20 years ago games came on floppies and ran on DOS. If I want to play one today, the legality of my copy is really the least of the hurdles.
The reason I can still play these games is because I have a copy of them. That's the point in question here. Do I really have a copy of Portal 2? It's on my computer now, sure. But will I be able to fire it up and reminisce in 2031?
To me it's a bit like suggesting that you should just photocopy a book rather than buying it because if you buy it the pages may wear out one day.
Can I definitively say that this will hold for all my current Steam purchases? No, of course not. I can, however, say that Valve has demonstrated the ability to keep giving me access to my games over a period of more than a decade.
In contrast, I have many games on CD that I can't play any more, because they don't work on modern operating systems. Some of these I've repurchased through gog.com, just to get working versions. Some I've repurchased even though they still work, just to get versions that don't require five CDs to install.
Buying a game from Steam is a calculated risk. It comes with advantages: No need to fiddle with discs, automatic patch updates, and access to all my games from any networked computer. It comes with one big disadvantage: The potential of losing access to my games at some unspecified point in the future.
That's a pretty good tradeoff for me. Maybe it isn't for you, and that's okay. When I consider Valve's established track record, however, I believe that the odds of Steam providing my games for long enough are very good--where "long enough" is "until the game ceases to work on newer operating systems and/or becomes available for repurchase for trivial amounts of money".
And if they fail me, there's always piracy. After all, I'm hardly going to feel guilty or morally culpable for resorting to piracy to reacquire access to something that I already own.