Believe me, I used to get in just the same frame of mind over the thought of films just disappearing
I still enjoy a good rant about buying 4K stuff that gets downgraded to HD, as far as I'm concerned I bought 4K, I expect it to play in 4K, despite Apple saying we technically only bought HD, even though they're advertising 4K
As I said, to the best of my knowledge, none of my content has permanently disappeared, so I don't worry about it.
If, as you say, 100s started disappearing overnight, then yes, that would change things and I'd certainly stop buying new content.
That's as likely as my house getting struck by a meteor and wiping out a few hundred of my disc collection, I'd probably have the same recourse with that. I wouldn't be able to provide the insurance company with a comprehensive list of what I'd lost, nor the cost, which would've been considerably more when I bought them than they are worth today, so I'd likely not recoup anything like enough to replace them, even if I did know what was missing.
No, films shouldn't disappear permanently from our libraries and in practice, except in very rare exceptions, they don't.
If Apple get to the stage where they're going bust, I'd back up my collection. Don't see that happening anytime soon though. Even if it did, I doubt it would be literally overnight, with no warning.
At the end of the day, it's something that you just have to live with if you want to go digital.
What's the alternative? Spend a hell of a lot more on discs.
What happens to all the films you might own when you can no longer buy a VHS player, or DVDs if there comes a point when you can no longer buy a DVD player? Doesn't matter if you've still got a room full of discs if you can't play them.