While I do not claim to know much about this discussion, as well as believing you have a very valid point, I just want to add an additional information:
While it is true that (normally - not always), platforms with drm (meaning they actually keep your games and you cannot play them without using their services) will keep your access as long as you have purchased a game before it being removed from the storefront, the very fact that itch.io offers games without drm means that you 'technically' do not need such a thing. You can literally, after you downloaded the games, keep them forever - standalone - to be used, backed up, and - although you are usually not allowed to share them publicy, give them to your close neighborhood friend like in the good old days.
A combination of both would be great of course. I do not know exactly how gog handles it, but they do offer drm-free games you would at least assume can still be accessed from your library if you have already purchased them. In that regard, itch.io does have restrictions to delete projects and accounts if people have already bought your work, so to enable them to redownload the items. But I assume in very specific cases like this one might be, there is not too much the store can do if a readily available file gets removed from the platform. But that is already going into a territory I can only make assumptions, so I will leave it at that.
Also, while I do see a point discussing this (and being aware of it for the future), one does have to acknowledge the fact that this is quite a specific scenario. Not every game franchise will make some legal deals with streaming companies. But it does indeed shed some light into the questions of this being only one example of it being too easy to pull things from buyers without 'fair' reason (legal does not always have to be that), as well as the thought of some sellers/creators not caring too much about the consequences of leaving behind a certain platform/group of their users for debatably justified reasons.