Steam's DRM is so harmless and unobtrusive that it's not worth discussing unless you have a telephone pole lodged in your nether regions.
Buying a game on GOG doesn't guarantee there will be no DRM - there just won't be any of THEIR DRM on it. Companies can still require you to login and/or jump thru other hoops that are DRM.
I play games that are 20-30 years old all of the time and I can tell you that virtually all the companies that made them are gone or have been bought out. Steam going out of business in the next couple of years is a remote possibility, but you never know.
I think it is far more likely that they get bought out. Gabe isn't getting any younger and he might want to cash out. EA, Ubisoft, or Microsoft etc would love to purchase steam and would then have control over your entire steam library.
Yes. You should form opinions and how you roll over wild fantasies and complete zero-fact speculation.
It's equally likely that GOG folds or gets bought out. Maybe even more likely since they don't have the same type of community setup. If you're going to make up fantasies you may as well make them plausible.
Plus, I'm sure you could actually do some research to see why gaming companies 20-30 years old get bought out or disappeared.
Back in those days so many companies making games were basically owned by publishers/corporations and so were the games they created, and they went under because they couldn't make money selling their games, and they didn't sell their games, because they never owned them, the publisher/corporation that only cares about money sold the property.
These days it's far more likely for non corporate slave game creators to not have this problem. Indie teams are more common, crowd funding, massive exposure thru Steam without having to do a ton of marketing, early access thru Steam, better coding tools, better distributed work conditions (working from home/remotely), etc.
That's not to say games won't still be bought and sold. Lots of successful entities change hands over time. Business is business and money tends to trump everything much of the time.
Gabe and Valve are very successful. Ridiculously so. Steam is a cash cow. Why would he cash out to get more money he'll never spend? Steam is so stupidly successful that Valve can piss away money on silly ideas like Linux-based consoles (in an already flooded console market and to compete against the platform (PC) that made them, which is pretty stupid) and VR toys.
I don't think Steam would have nearly as much traction if it was owned by MS or EA or the like. I'm a MS developer and generally like MS but I wouldn't want them holding my game library and surely wouldn't want EA holding it. MS/EA may fantasize about owning Steam but do you really think people would want to be involved with Steam as much as they do now if either of those owned it?