It would be reasonable to expect the train to be held.
However, no amount of arguing about it will change the fact that it won't be held for you in these circumstances. If one were to imply to your wife that it might be held then I'm afraid that would be setting a very unrealistic expectation.
If that means that your wife drives instead of paying to use the train, the response from the railway industry is, currently, so be it.
In any case, the platform staff themselves have no particular influence over what happens anyway. Your complaint needs to be taken up with management, but it is very unlikely to reach th by speaking to station staff or writing to customer services. If you get in touch with your MP's office, and ask them to challenge the policy at CrossCountry or GWR on holding train connections on your behalf, you may get further. But equally, don't expect anything to change as a result of one complaint.
And this is why many of us use the car not get the train. In a changed situation where the railway cannot rely on commuters (with no alternative) maybe the thinking needs to change.
Best advice to the OP is, should the same issue occur again, ask the guard on your initial service if they can request a hold. Staff do not do this sort of thing automatically. A train coming into Bristol from the 'Beach in the AM peak will likely have passengers for an array of destinations. There will almost certainly have been folk aiming to make the 8.30 Paddington train, plus a whole host of other connections, valid or otherwise. It simply isn't feasible to be guessing at people's intended journeys and holding every train in Bristol at half 8 in the morning just in case. As to the platform staff, they are kept very busy indeed at a station like Temple Meads, besides dispatching they are continuously fielding enquiries from folk already on the platform. It isn't their role to monitor every train in and out to see what's on time and guess at which trains to delay for connections.
You said that your train was delayed due to the service heading in the opposite direction running late. And there you have the perfect example of how delays can spread across the network, and why it isn't a simple case of holding on to a train for a couple of minutes. Had that other service not been delayed, and who knows why that was, your train would have been on time. Had the XC been delayed to wait for your service, that is one late running service turning into three. And so on!
So.
The passenger has first to realise themselves the train is late compared to its schedule down the line from where they boarded, have an understanding of any recovery time or not, etc.
The passenger then has to take it on themselves to go and find the guard, who with units multipled together may be wholly inaccessible.
The passenger also has to know their guard could do something about maintaining a tight connection, a service which is not advertised anywhere.
The guard then has to try and get through to Swindon Control and request this.
In this case Swindon Control now has to try and get through to Cross Country control in Birmingham, because apparently despite GWR being in charge of dispatch at Bristol, they are not - if it's a different company's train.
Cross Country Control then has to try and get through to someone (their guard?) to say hold the train.
That's not going to all work in a few minutes, is it?
Let me tell you how it used to work in the 1960s on the Taunton-Minehead branch. The guard would notice passengers joining the train at Minehead with heavy luggage, or chat to the porter, that there were passengers connecting to The Devonian at Taunton. If delayed on the branch, the guard would shout to the platform porter at Bishops Lydeard (notice proactively, doesn't have to be told) "Tell 'em to hold the Devvy a bit, people for Leeds". The porter would phone directly to the Up platform inspector at Taunton (notice trusted to do this directly, no need to go through two levels of Control), who would send a porter down to the branch bay to assist with any luggage. All in, off a few minutes late. Full regulator, fireman shovels a bit more, back on time at Bristol. This wasn't Thomas the Tank Engine, it was how things were actually run then.
If there's connections, it shouldn't be up to the passenger to try to manage problems....... and if the connection is always tight/incoming train late a lot then the train planning needs looking at.
The irony would be that if the OP explained Delay Repay to his wife and she still chose to drive in future, only to be delayed on the motorway. Could someone remind me how much compensation Highways England pay drivers for delays caused by roadwork, accidents, et al?
I drive a lot and it's rare to have a delay that you were not warned about- the roadside dot matrix signs, travel news bulletins and the Google Maps live traffic feed all work well although of course you do get "ramdom roadworks roulette" once the hour of 20:00 strikes.......
In any case, being stuck in a warm car with comfy seat and music (or in my Big Van with full facilities for a brew, loo etc) is a mile better than being stuck on a cold metal bench on a dark windswept platform at night with the waiting rooms and toilets closed.
Most people who travel a lot just want to get to where they are going, and in general would prefer a comfy reliable journey rather than compensation for a delay.
If we want to attract people out of cars onto trains we need to be identifying and solving some basic issues like comfort, connections and customer service- not making excuses.
TPO