I'm talking more broadly about the types of things the more average users need and expect when interacting with both an operating system and the software on it. It's easy to think that most people are at our level of knowledge about computers and applications but I think the reality is far different, even considering the Steam user base is comprised of PC users. The trouble with package managers is that people need to have an idea what they're looking for and won't stray far from familiar names and brands (Microsoft Office, Adobe Photoshop, etc). We have a tendency around here to think every user is at our level but I would wager the majority of Steam accounts are not enthusiasts with the level of familiarity that would be required to maintain and potentially troubleshoot a Linux installation.
I think Ubuntu especially has made tremendous strides over the years and I run both Linux and FreeBSD systems but I'd be pretty wary of installing it for family again. The last time I tried the cheap Linux machine route, I setup a theme that was essentially Windows throughout and thought I had foreseen every possible problem. In the end I ended up back at the relatives place because they couldn't figure out how to attach files to an email because the process was slightly different (unfamiliar filesystem, etc).
Anyways to be fair this isn't happening in a vacuum, Valve can slowly transition people away through word of mouth and etc. If anyone has the swing to get other devs on board its Valve and there's a whole new generation of devs who aren't dependent on publisher backing. We're also less dependent on PCs than we used to be thanks to smartphones so maybe people could get by without installing XYZ big name commercial app. Either way it'll be interesting to see how it shakes out.
Playing: Fallout 4, Warhammer Vermintide Watching: Ash vs Evil Dead