I've met a significant number of younger enthusiasts who have no interest in steam whatsoever, regardless of what colour it's painted. It just isn't relevant to them. Given that Railways are far less of a mainstream interest than they were for the previous few generations, there will be fewer interested people to potentially preserve more and more items, as not only do we wish to continue with the preservation of all the existing stuff, but we'll have to add many of the current locos / stock / .... to that list. Something will have to give, unless railways suddenly become an in-vogue thing to be interested in and there's a mass influx of people. There appears little chance of that being the case with the current generation, by which time it'll likely be too late for a good number of preserved items.