The reason why 10 year old steam locomotives were scrapped was simple - they should never have been built in the first place.
Oh, I can't agree with that. The railways had a motley collection of largely clapped-out traction. New and standardised locomotives were needed, diesel traction was not well established at the time, diesel locomotives cost about three times as much as steam if not more, and there was considerable uncertainty over how long it would take to be brought in and also to what extent
vice electrification. To meet the immediate and urgent need it made sense to use the established and cheap technology for which all the infrastructure and skills were already there.
What should never have been built in the first place was the motley collection of largely clapped-out-from-the-word-go diesels that resulted from the rush into dieselisation after those in charge had somehow got the idea that it was a magical solution to everything when it was still an experimental technology. They should have stuck with the original, sensible idea of buying 10 of each of a variety of designs and then waiting to see if they were any good or not before deciding which ones to go for. Instead they rushed hundreds of untested experiments into full service and hoped they wouldn't blow up, which was just dumb.
Add into it Western Region going for Diesel Hydraulics while everyone else was going for DE.
Class 14 - 56 - withdrawn in less than 5 years
Class 22 - 57 - withdrawn in less than 10 years
Hymeks - 101 - withdrawn 10-15 years
D600 - 5 - withdrawn in less than 10 years
D800 (Swindon) - 42 - withdrawn in 15 years
D800 (NBL) - 33 - Withdrawn 9-11 years
Westerns - 74 - withdrawn 12-15 years
Basically 360+ locos very few of which had a life longer than 15 years.
Well, there were good reasons to choose hydraulics. The WR already had experience of the electrical side of "DE", from their gas turbine experiments, and didn't want it. They wanted a proven design with a high power-to-weight ratio. An adaptation of the German V200 design to the British loading gauge was just the ticket.
Then the silly politics got in the way: the government not wanting to buy stuff off those nasty Germans who we'd just fought a war with, and insisting that people do something before they can be given money regardless of whether what they are doing is any use or not. So instead of using the well-made, reliable engines and transmissions straight from the German designers, they had to use the cruddy licensed versions built from inaccurately-converted measurements using substandard materials by a bunch of blacksmiths and men with big hammers, with NBL failing to realise that while you might be fine making a steam locomotive like this a diesel engine was a different matter.
They even had to accept a tranche of overweight, lumbering behemoths which NBL made by carving bodyshells out of battleship armour with a hammer and chisel and then panicking because they couldn't figure out how to make a bogie to take the weight, and sold to the WR at a loss hoping to make it up by selling more in future even though they sucked and the WR had never even wanted them in the first place.
The railways responded to NBL by insisting that NBL repair their own crap at their own expense and guarantee it for three months, in the vain hope that this would give them sufficient incentive to make the things properly in the first place. NBL responded to the railways by going bust. This then put the kybosh on the survival of all the hydraulic classes that depended on NBL being able to supply spare parts, which was about a third of them.
Also in the category of silly politics locomotives are the Class 14s, whose intended traffic was disappearing even as they were being built, but despite this they kept on building them, because
of course instead of paying people at Swindon to not build locomotives it is better to pay several times as much to have them building locomotives you'll never use.
So that disposes of about half the hydraulics episode as silliness that could have been avoided by shooting all the politicians, which is a good idea in any case, and leaves us with the Westerns, Warships (excluding those with NBL parts), and also the Hymeks.
The "Ws" were actually good locomotives for the most part, certainly no worse than diesels in general, and a lot better than the Peaks - a rather less advanced design - were managing at the time. A lot of the reason why they were underappreciated is not inherent in the designs themselves.
They were handicapped in their use for freight by lack of brake force due to their light weight. At the time they were ordered this wasn't supposed to matter because all the unfitted freight vehicles were about to be scrapped anyway. In the event it took us forever to get shot of the things and the hydraulics were just one unfortunate casualty of the authorities getting their priorities upside down.
Part of the trick of the high availability obtained by the Germans with the original design was unit replacement. Engines and transmissions could be removed in their entirety and overhauled as a background process while the loco was fitted with a spare unit standing ready and put back into traffic without delay. To further facilitate this process, the two types of engines and two types of transmissions were all interchangeable. The WR intended to do this but never got it together and nearly always just patched things up in situ, which was particularly difficult given the lack of space for access. Not to mention that NBL buggered it up by not putting the mounting points in the standard places and killing the interchangeability aspect.
And then there was the steam heat fiasco which did for a whole lot of diesel classes both hydraulic and electric. In theory steam heat was a great idea as it allowed the continued use of existing carriage stock, it didn't reduce the available power for traction, it could (in theory, though they never did it) be produced for free using the waste heat in the engine exhaust, and it was lovely and cosy - when it was working. The problem was that it so often wasn't, because not one of the different manufacturers of steam heat boilers knew a bolt from a rivet and every design that any of them supplied was a useless heap of junk that broke down more often than all the other bits of the loco put together. And the hydraulics' lack of interior space made it hard to engineer an ETH conversion because there was nowhere to put the generator.
Running through the whole saga is the thread of the WR not realising that they weren't the GWR any more and all the bitching back and forth between them and the overall management, which didn't help anyone or anything, and the use of the "standardisation" excuse to provide a stick to slap the WR down. Given the largely self-contained nature of the hydraulics' duties on the WR and the existence of the WR's own shops at Swindon to deal with heavy repairs, it's not really such a big deal. Any attention they might require at other regions' depots would more than likely be to parts which were common to many classes, and not to the unique transmissions.
The hydraulic transmission was a good choice. Only a very small proportion of breakdowns were due to transmission failures, and most of those were in the nature of teething troubles. This was not the case with electric transmission. The hydraulic system also gave better traction for starting heavy loads than was achievable with electric transmission before the development of good electronic wheelslip control systems. Also, the redundancy in the duplicated engines and transmissions gave better failure resilience.
If the WR had been allowed to crack straight on with the "stunted V200" design, using German-made components, from the word go, instead of messing about with the NBL junk, and had had the time wasted messing about available to sort out the teething troubles and get the unit replacement system up and running, then I think the story could have been very different: the WR would have had a very noticeably lower proportion of dross in its loco fleet throughout the sixties compared to the other regions, and with the principle thus shown to be successful it would have been natural for hydraulics to become favoured over electrics more generally.