I'm not sure that air-con stock itself has effected bookings to a massive degree (though I reckon it will have done to a degree as I do think Mk1s with opening windows make for a better experience overall) but I do think the cost of actually doing a railtour will have done.
For instance UK Railtours Kent Coaster in early June - Standard class £129, First Non-Dining £179, First Dining £289. The BLS don't seem to have any mainline tours open for booking at the moment but they're usually in a similar ballpark to UK Railtours (though without dining). LSL's Tor Bay & Dart Express in July Standard class £119 and First Non-Dining £149.
Then add in probably at least one night in a hotel which is often going to rush you the thick end of £100 for a Friday or Saturday night and then travel to get to the start and back again and I reckon you can easily be looking at dropping £300+ for a railtour these days even in Standard class. That's a lot of money. It wasn't all that long ago that you'd get teh tour ticket for £80/90 and the hotel for £50. Considering all the other cost of living increases that are going on in the background I reckon that a lot of people are being far more selective. I know I am!
My loyalty is pretty much just to the BLS these days as I know that they'll always put on a good tour and anyone else it basically has to have some sort of real key highlight (very interesting traction, interesting track, interesting people to tag along with!) to get my interest. Otherwise it's just so much money.
I'm sure that enthusiasts and others have always whinged about the price of railtours (I think Pathfinder have been known as Chingfinder for a large number of years?

) but even in the time I've been doing railtours (around 15 years now) it feels like they've gotten more expensive in a fundamental way which impacts on the ability of certain chunks of the target market to actually do them, or at least as many of them, as it used to be possible to do five or six years ago.