If that were to happen to the 59 it would in effect just become a short version of the 159, a very poor result considering the success of the 59 in bringing new custom to buses from the Streatham/Brixton areas: if anything, it should be re-extended to Kings Cross, but it won't be. The 468 could be extended from Elephant to Waterloo and the 68 withdrawn. Personally, I'd then renumber 468 as 68 and introduce peak shorts to West Norwood (with Hopper fare there really should be no reason why scheduled short workings continue to be outlawed by TfL.) Consideration to introducing a between peak Monday to Friday service on the X68, every 20 minutes, would also be part of the package - I see no reason why this wouldn't be a success.
Yes - the 59 goes to places that none of the other Holborn-Waterloo routes do ... it should be the last to go if de-duplication is the idea. And the 59 cut-back from Kings Cross to Euston has now removed the only direct public transport link between Waterloo and KX/StP - so yes, it should be re-extended.
The 'consultation' on the 168 ends today so that may go this year and be merged with the 1. As the 59 and 68 both terminate at Euston , if terminated at Waterloo together as well then one of them may as well go too and be merged.
The idea seems to be for the 1 to take over the 168 route in the north and centre, with the latter being scrapped, which means that some places at the very southern end of the 168 route will no longer have their current direct service. That will also remove the last bus from Waterloo Bridge which turns left at Holborn to Tottenham Court Road (ie the last northern part of the existing 1 route); to deal with that, they're
also proposing to cut back the 188 route from Russell Square, and send that west from Holborn instead - hence reducing even further the number of buses covering part of the major north-south route there.
There are so many places around London where these sort of simplifications could be made to reduce duplications and running miles that cutting 100 routes may not be the disaster it is portrayed as being.
What is a "simplification" to a well-heeled planner looking at neat lines on a map is the exact opposite - a complication (not to mention a significant slowing of the overall journey) - for someone struggling (on account of disability, luggage, kids, shopping, whatever) to make a journey on a wet winter evening. Those "simplifying"/"rationalising" public transport routes rarely seem to have any real-world experience of the struggles of some of those relying on the services.
I worked for both LT and TfL and as far as I'm concerned it only started going wrong under Johnson. Prior to that were the massive improvements to the bus network, the inception of London Overground and Congestion Charging. He cancelled the western extension of the Congestion Charging zone to please his Borough mates, cancelled the cross-river tram, halted further expansion of DLR and Tramlink and instead wasted money on that stupid bus and ill-conceived and rushed cycling schemes. Unfortunately the current mayor has not been wishy-washy at best.
The "stupid" bus was one good thing that happened in that era - while the rear platforms were open, it cut up to 30% off of many end-to-end journey times. But of course bus passengers are not considered important enough for the value of their time to be included in any cost-benefit analysis. The cycling schemes haven't been all bad either - mostly they've been too unambitiions because of fear of alienating car drivers (who shouldn't be in much of London anyway).