If a big player like Steam [...] became interested in creating a side business that pushes DRM-free gaming, then it would become popular.
Nah, sorry, but I don't see that...
Now - would "DRM-free" have a chance to get more popular than it is now? - Maybe (and that's a huge maybe)
But then only due to the bigger numbers of customers that it would potentially(!) reach.
However: a Steam backed "DRM-free movement" (in its most strictest sense)
would still come with all the downsides that "DRM-free" on GOG (or "DRM-free" in general)
comes with, so why would Steam customers care for it?
For them, a "DRM-free-Steam" would just be Steam with lesser features and less convenience
That's not really much of a selling point.
To try and bring the conversation back on track
Steam backed DRM Free, would still cause the same issue GOG has getting developers on board.
This whole Steam vs "DRM Free" conversation is a distraction from the main issue which is
Company B can't get Devs to release their games on their platform because Company A is so dominating it makes releasing on Company B financially unviable for the Devs, which in turn reduces Company B's ability to compete and further strengthening Company A's domination.
We've ended down the conversation route of "But people CHOSE Steam", which an objective look at the history of Steam its actually "People chose steam over not having any of the games they wanted to play" rather than "People chose steam over some alternative way of legally playing the games they wanted"
Valve has had market domination for over a decade, the vast majority of popular games are exclusive to Steam. The PC gaming market was not competitive. GOG built its position from a niche, Retro and DRM Free, but without a guaranteed stream of timely released popular games that Niche will only get them so far.
The situation of the Market is far better than it was 5 years ago, we're slowly seeing viable competition emerging from the likes of Epic and Microsoft. Though its now more segregated, with more companies having exclusive titles.
GOG is in a precarious position, with many devs now choosing Epic as their "alternative" to Steam where they once might have gone on GOG.