Part of the problem with steam, even though it is bound to be popular thanks to society's general liking for nostalgia (and Thomas the Tank), is that the locos themselves will be increasingly hard to keep running. Then there's the question of how much will be left, or will we be in a Trigger's Broom situation, where frames, wheels, boilers and everything else will have been replaced at some time? It probably won't happen for a while, but the diversity of classes will go down, and the harder ones to maintain will gradually end up on static display only. Then of course there is the loss of skills. The old BR steam men will no longer be with us, and where do you find people to take over? There will be some, but not so many.
It's possible there will be new engines, new-build Standard 3 tanks are one possibility. Being easy to maintain and with no original parts, they could keep going as working engines as long as there is still coal to burn. I hope that that will be when I am long gone.
I think the situation has already moved on from that which you describe, and that the preservation movement is well aware of the challenges it faces, and is often acting accordingly. Commercially, there is simply no choice but to keep steam operational, as it is what the general public wants. 25 years experience tells me that if they see a diesel, they frequently either wait for the next steam service, or ask for their money back. And it is the general public that bring the money in.
Steam locos are very simple beasts by their very nature. Patterns for castings exist for many common components. Extensive rebuilding of boilers is now regularly taking place. They've just steam tested a boiler at Grosmont over the last week that has had a virtually new barrel fabricated in house, as well as extensive firebox renewals. That's after the Black 5 with a 90% new boiler last year. "Trigger's Broom" maybe, but it is already happening.
BR steam men are increasingly a thing of the past. The youngest passed cleaner is about 63 - 65 by now. That's people who managed a couple of years on the things at best. Plenty of lines are running apprenticeship schemes, for young people to learn the traditional skills. Not as many as would be ideal, but a move in the right direction. The boilers I mentioned above - overhauls masterminded by a bloke who's not 30 yet, and who trained in house. There's plenty of young blood wanting to learn the footplate side too, and more is always welcome.
Replicas steam locos are a bit of a red herring. A full rebuild of an existing loco will cost about 500 - 600 k. A new loco is going to be 3 - 4 times that, so that's not going to be the route the railways themselves take, and it is increasingly they who own the locos directly now).
Components for diesel classes are far more complex, and far more difficult to manufacture in house. As the supply of them dries up, it will actually be far harder to keep the more modern types in service. 2nd and 3rd generation varieties will be harder still, due to the increasingly complex electronics involved. With a steam loco, if you can't easily get a particular lubricator, ejector, injector, etc, it is relatively easy to adapt something else to do the job. With an electronically controlled modern diesel, that will be far more difficult. The latter will also not be particularly happy in the typical open storage on a preserved line. The same is true for later MkII and Mk III coaches. Electronics again.
Furthermore, most of the major lines have already got the "full" signs out. Where are you going to keep the modern stuff?
