Memecchi: Yeah no, that's not how it works at all for most people in practice, if someone has an steam account, they'll try to buy all of their games there, in fact a lot of steam users avoid GOG simply because they already use steam, not because they have anything against GOG or love DRM
It's just more convenient to stick to one platform
Nah, that's how it worked years ago, all a bit different now.
Epic for one, have made sure of that, plus GOG with exclusives, and a good number of younger folk are far more flexible, and sales price can often be everything.
So you are really only talking about the old Steam diehards, which are for sure a huge number, but there would also be a large number who aren't like them, who don't necessarily care about DRM-Free, but do like to shop around, try other things.
P.S. Just like many don't stick to old Netflix, but often have Prime instead of or as well as ... and some others.