I’d go with a simple proposal: if you use the word “buy” the seller is required to refund buyers if the seller’s action removes access (unlike, say, losing access to a CD or DVD after the disc is scratched).
The most likely outcome is that they’d change their storefronts to use the word “rent” but that’s a good outcome (it encourages buyers to accurately understand what they’re paying for) and it would allow other options like releasing an old game without DRM prior to killing servers.
I agree, the "buy" on a streamable product implies that as long as the company is solvent and they have the same overall streamable products available, then your "purchase" should always be upheld.
In a an ideal world, if you bought the title then you'd have the ability to download it locally. I understand the licensing is a main driver of taking titles down, but these agreements are completely opaque to the customer at the time of purchase.