General aviation has been on a slide for a long time, and, indeed, a core mission of the AOPA has been to increase interest in GA. Witness what's happened to automobiles. Sure, the American passion for cars hasn't really diminished, but a manual transmission has become the new anti-theft device. People can't read maps anymore because they depend on their smart phones to do their navigation. My own son loves his Tesla with its auto-driving capabilities. I suspect auto-flying aircraft taxis will become a thing if/when the technology becomes practical, so that might increase people using GA, but fewer and fewer people will want to accept the challenge (and risks) of flying little airplanes like Cessnas and Mooneys when it does.
But back to Cliffy's point (which I agree with), maybe we can look to something like the Navion community. Only 2634 total were ever built; less than 200 were built between 1960 and 1976 when they stopped production. And yet there's a pretty strong community keeping these birds in the air. There are a couple beautiful ones on my field with maxed out panels and show paint and interiors, probably in the $200K value range. Why? Because they offer something unique and offer a high-end flying experience at an "affordable" price. Our Mooneys will slowly tread the same path. They are wayyy more affordable than just about any new light plane. Even the last production Mooneys look pretty cost effective compared to a new Cirrus. And the old ones look very attractive. The humble M20C is a serious competitor to a Cirrus SR20 at a fraction of the cost on the used market.
So I expect our fleet to dwindle as Cliffy suggests, but I also expect them to continued to be highly valued and increasingly updated as the years go on. I'm already seeing the trend, as more and more 'humble' Mooneys get new paint, new engines and new avionics. There are a couple of M20C/D/E/F/G models with killer panels in the $100K - $130K range on Trade-a-Plane. Yes, right now those are 'expensive' (and probably slow to sell), but their owners decided to update them because they wanted the performance those upgrades offered. No, they aren't the bottom-of-the-barrel bargains, but they are and will be serious competitors compared to more 'modern' airplanes as time goes on.