On the market value aspect I'm not sure Cal44 is right about the peak, but he's not far off the mark. I'm 65 so the 60s made a big impact on young Brian, Emma Peel in her Lotus Elan being one of those factors. My generation has retired and got cash to spend, hence the market bubble in Elans with Europas being dragged slowly along behind. I can see it stabilizing at some point as the next generation moves in with it's favourite cars and we're all to old and creaky to get in ours.
But people over here still drive around in Fords from the 20s/30s, and if you want something to compete in the London-Brighton Rally (Vintage) then you need deep pockets, so a market will always exist. I'm more concerned about the market value of our moderns because they are the ones we'll be selling, the Lotus's are toys and you never sell good toys.
As for spares, I think we're in a golden age at the moment. Back in the late 70s UK getting parts for any 60s Lotus was hard work, Lotus had nothing and hence specialists like Christopher Neil set themselves up. I think Richard came on the scene around that time as well.
I'm amazed at some of the parts which are available now, but as you say the current crop of specialists will eventually retire. Maybe they'll run as a cottage industry like Sue Miller does, concentrating on scarce parts with quality foremost or perhaps they'll sell up to the next in line. Paul Matty has retired now and someone else (can't recall) is now running his old business. If there's a market then someone will provide for it.
Worried ? No. My eyes have been opened by some of the folks on this forum. Rod comes first to mind with his creation which is really hand crafted from basic materials, and that's encouraged me to branch out. I would never have considered changing the OEM brakes on my car in the 70s but now I just think "what's the best solution here ?". And Alex with his yard full of restorations, sourcing parts and skills such as the latest stainless headers & Al seats he's obtained. The cars are simple and I think I'll be able to keep it going for as long as I want it. But a 2010's car ? no, I don't want to keep one of those long term. ECU interfacing with alarms ? circuit boards in an engine bay ? eh ? built in obsolescence.
Future plans ? no, not really although I do keep my eyes out for an NG3, just in case. But if I could no longer drive it I'd probably just polish it and sit inside making brum-brum noises