Even if it isn't, old games eventually just stop running as Microsoft releases newer versions of Windows. So, even "owning" the install files is not a long-term solution. I think we should accept the fact that video games are just ephemeral goods that disintegrate within 5-20 years timespan.
I just need to comment that this is alarmist and ridiculous. I have had no issues running old applications on modern versions of windows whatsoever.
If anything, I think it's particularly impressive that Windows has as much backwards compatibility as possible going back to some of the earliest popular versions of windows.
At most, you could argue that BASIC and DOS are the biggest examples of us losing compatibility but realistically noone cares because they can be emulated at speeds a thousandfold the hardware they once existed on.
The most terrifying part about steam is them banning you arbitrarily or your account being hacked.
That's the real concern not some kind of compatibility nonsense.