I appreciate that fact that you added the word 'generally' however, there are exceptions and I very much doubt I'm the only one. I'm not old enough to remember mainline steam but I went to school and college in the BR corporate blue era and as much as I love railways (I worked for the rail industry for twelve years) I have less than zero interest in recreating the railway of my youth or indeed my later life.
The steam powered railway of the 1920s and 30s has been my passion but now increasingly it's the pre-group era, the golden age of steam, that really appeals. What may actually happen in time is exactly the opposite of what you suggest. As the population ages and fewer people can recall mainline steam, like me they won't be modelling the railway of their youth at all. Given a free reign without personal nostalgic experiences to steer them they may choose to model any era and location they fancy. That the pre-group steam engine models have proven to be strong sellers indicates that there is a latent interest in this era of railway history and that may gather pace.
The point is that we just don't know what the model buying public may primarily want to buy in 20 years time until we get there.