Even on Steam there are already games (and I'm talking about actual good ones, not crap like whatever Digital Homicide shat out) that are no longer available for purchase, for whatever asinine reasons.

As one example, Wallace & Gromit's Grand Adventures from Telltale Games. This is because of an "expired agreement" between Telltale and Aardman Studios or whatever dumb bullshit. These games don't even have Steam pages at all, anymore. I still have them in my Steam library for now, but if Steam ever decides to no longer make them available for download, there is basically shit-all that I can do about it.

Here is a list of other titles that are no longer available on Steam, for a variety of reasons. I'm not sure how complete it is, and some games are on there for other reasons (like the fact that they apparently were never released despite being listed on Steam), but even if you hide everything except for "Delisted" and "Purchase disabled", the list is still depressingly huge. Granted, too, some of those were delisted because they were replaced with "definitive editions" or whatever, but many, like the above mentioned Wallace and Gromit games, are just straight up gone.