Presumably the market for people to gawp at elderly steam locos is finite
Ignoring the offensive sarcasm people will always want to witness, admire, learn from and respect the skills of past generations. After all "elderly" and other folk still "gawp" at a pile of old stones called Stonehenge, and how old are they? ( the stones ).
Market forces and demand will determine if we have too many heritage railways and thus some will survive others will fail. It is no different to any other commercial or industrial enterprise. Without doubt increased costs of transport will curb visitor numbers so that offering value for money will become ever more vital. Sneer at Tank Engine Thomas if you will but it puts bums on seats.
What is unique to heritage railways and aviation is the curse of the freeloaders. Those who sit on fences and get in your way whilst they watch and photograph but rarely put their hands in their pockets or lift a muscle to help.