I thought about the "What if I had done something else for that same amount of time" thing before. The thing is we could tell that to ourselves regardless of what we spent that time on, but it's indeed very easy to think that playing video games instead of doing 'x' is a "waste" of time. In the grand scheme of things I think as long as you get by financially then you should have the right to play video games for any amount of time you want, be it on Steam or just in general.
In my case, as far as Steam goes I'm not sure. And in general... I started at around 8 or 9 years-old (I usually go with nine), a bit late actually compared to other gamers I spoke to about this before (whom in most cases told me they remember playing games from as soon as 5 or 6 years-old). I pretty much never stopped from that point, not a single year went by without playing games. In fact, close to not a single month went by without playing at least one game. The number of hours dedicated to video gaming would be astronomical for a human life span, all things considered. There's no way to know for sure, but I can for example just take specifics into account, such as Diablo 2; a game I know I spent about 6 years of my life playing. Obviously, it doesn't mean 6 years in a row while playing 24/7. But 6 years where pretty much every single day of weeks, and week-ends, give or take some exceptions here and there (very few) I would be playing it anywhere from 2 to up to 7 hours (even cutting on sleep hours to play it). The time I spent playing that game in and of itself would have probably given me nearly a full year worth of income if I had spent it in a full time job instead (all compressed time anyway, but it's just an estimation; might have been more actually).
But to come back on my point in the first paragraph, we're talking about video gaming but really... we can say this with anything else. If I spend 10 years of my life doing mountain biking many hours a day (in younger years anyway) I could easily stop and think "What if I had spent that time doing something else?". It goes with almost everything. Now, obviously, video gaming was never meant to be a source of income originally. So of course playing video games was perceived as a "waste of time" for... well, for a very long time. It has only been very recently in video gaming history where it's now possible to "not really waste your time" playing them anymore since not only is it possible to literally live off of doing that now, but you can even have a very comfortable life doing so, if it works for you anyway. With social media, monetization and streaming... people today can walk down a street with their cam Live and speak about how their bowl of cereals was good this morning while receiving donations and looking at "Chat" playing with emoticons in the meantime.
However, in some of those cases the "gamers" (or streamers or... "content creators") don't like it and it shows. They do it for the money. Good for them, we all need money. But would they do something else for that same money? Probably. Do they wonder if they're wasting their time and "What if..." when they think of what would happen if they had spent that time doing something else? Maybe not (we tend to not wonder about too many things when we have no financial worries in life). Anyway, speaking for myself, no regrets. Because I know I like it. I liked it from day one with my NES, and I still like it today on PC. I find it hard to have regrets about doing something I like. I could of course regret not playing games per se, but regret having spent so much time doing that. Then again, I might have regretted it if I had spent the exact same amount of time doing something I didn't like even if it meant having money from it (or "more" money from it) by comparison. I was never paid playing games, ever. In fact, like most of us, I have to spend money on this hobby. Still, no regrets.
The only way I would have regretted it (speaking for myself here) is only if after 'x' number of years playing games I would have came to the realization that I didn't like it, that it was a waste of time, that it didn't "bring" me anywhere, that it didn't "advance" my life, or myself as a person and that I should have done "something else" instead (even if it means blindly thinking that "something else"' anything else, would have automatically been "better" for me, without actually knowing if it would have been to start with). But the simple reality for me is that playing video games (or, well, precisely I should say playing video games I like) bring me joy in an otherwise very boring life. Outside of gaming I would have probably spent a similar amount of time either watching movies, reading books or drawing if I really had to do something else to entertain myself (I still do those other things but not as much in a dedicated way and sheer time as I do with gaming).