I think you’ll still be able to find 1 1/8 th forks second hand for a while yet, and the internal parts to keep existing ones running will be around for a while too, you just move on to looking at old stock or 2nd hand instead of new kit.
Plenty of options for square taper cranks, new old stock, posh stuff like middleburn, and then good old decent condition second hand.
7 speed not so much… but then 8 or 9sp upgrade is well worth while and plenty of parts still around. And even if you are hell bent on using and old 7speed wheel you just ditch the largest sprocket on the cassette and it will fit, and being that 8/9speed normally use 32 or 34 top cogs which were never available on 7speed you’re not actually losing out.
most older frames can at least accept new drivetrain standards which allow you to upgrade them, and as I said, I don’t think the fork problem will really be a major one for another few years, we’re only now at the stage where it’s almost impossible to get 1 inch forks (although there are still options 2nd hand for older Pace and Marz forks that work and are the right length for bikes of that era) and 1 inch stuff was pretty much phased out in the mid 90’s so that’s a pretty long innings.
I wouldn’t worry about it that much, by the time it is actually difficult to keep bikes like that running with decent kit it will be properly worn out or will have snapped.
I’m more worried by flash-in-the-pan standards that might leave people trapped, look at what happened to 1 1/4 steerers?