Yes, I think we've reached "peak preservation".
A few reasons:
The preservation movement did well when there were many abandoned lines that could be snapped up without much upgrade. That's not the case now - roads have been built in the way, retail parks built where stations once stood - there's not the same room for expansion or new lines that there once was
The preservation movement did well when many locomotives that had been taken out of service prematurely (e.g. a lot of steam locomotives were scrapped because they were steam locomotives and BR were rushing to diesel, rather than because they had become life expired, so they didn't need massive overhauls - plus BR didn't use stock as intensively so some locomotives were used fairly sparingly - compared to the outrage that you now get if someone sees as much as a two coach DMU sat unused at Tyseley/ Heaton/ Neville Hill off peak). That's not the case now - any steam is several decades old and any diesel stock that gets withdrawn nowadays has been fairly intensively worked on the modern railway.
The preservation movement did well when construction could be done fairly cheaply - land was cheap - applying twenty first century regulations would have killed off a lot of the re-openings that did happen - it's a much more professional (i.e. "expensive") world now
The preservation movement did well when there were a lot of people able to retire in their fifties and sixties on good pensions, able to devote a bit of time to projects when they were still physically able to be of much use. One of the older members of staff at the place I work left with the intention to volunteer at Middleton in Leeds but his generation were able to - many of today's thirty/forty somethings will be lucky to retire much before seventy
The preservation movement did well when there was a connection with the steam that they ran - the "tourists" paid money to ride on the kind of steam trains they used to commute on, staffed by the kind of people who'd have been working such services back in the day. Now though, there's a disconnect - kids grow up with Thomas The Tank Engine and the Hogwart's Express but the trains they use for commuting are more likely to be DMUs - the staff working on preserved lines won't be working the traction they might have worked on as drivers/guards on the "big railway"
The preservation movement did well when there were fewer tourist attractions around. Nowadays every other farm seems to have converted a couple of fields to a big playground/ "tractor rides"/ "sheep racing" etc, museums make much more effort to attract people, we have Wildlife Parks and "Go Apes" and more old buildings opened as Stately Homes, the market for the "Family Friendly Day Out That Shouldn't Cost More Than Fifty Quid All In" has a lot of competition. You can take the family to the zoo and be looking at giraffes a couple of minutes later but you might take the family to a preserved railway and find that the next service isn't for over an hour (and it's set up so that 99% of the attractions are at the "far" station so there's nothing much to do at the station you parked your car at).
The preservation movement did well when there were fewer types of train - much easier to find something that would appeal to people back in the day but in 2021, do you focus on one type of type/era, do you offer a muddle of everything - how do you find a gap in the market when there are so many different versions of "railway" that you could try to recreate? Are you aiming for a particular niche of enthusiast or just ordinary punters who think that A Train Is A Train Is A Train (and wouldn't notice that you had carriages only built after the locomotive hauling them had been scrapped)?
Plus, as per the above comments, the money to be made from retailing and food faces a lot more competition - gift shops aren't the money maker that they once were - you'll still be able to make a good profit on a cup of hot tea on a cold day but people are more savvy when it comes to paying over the odds for a meal.
There's certainly a market for preserved railways but I think that some contraction will probably happen over the next decade (sure, there'll be some expansion in places but I think that there'll be more contraction overall - even if there are big noises made about ambitious expansion plans which never come to fruition). Maybe we'll have fewer lines but bigger/better ones, more focus on a wide range of things (e.g. more than one train in operation at once)