Steam has made the concept of a perpetual, one-time rental service palatable.
It's convenient... REALLY convenient, and it was the first in the market. That should explain the success. Nobody really cares about the whole "you don't own your games" thing, since they haven't been screwed over yet. Valve already won the battle with steam, especially if you consider the other forms of DRM out there. Valve's DRM is least intrusive, up to the point where Steams users are for the most part blissfully unaware that it is there. Origin is a bad joke where the audience refuses to laugh and owned by a company which has a terrible reputation, and the others are so insignificant that it's hardly worth mentioning them (Stardocks Impulse, if that still exists, springs to mind).
I cannot get Dishonored DRM-free - it's Steam or bust
That is in the end the choice of the developer/publisher to pick what platform they want to use. Don't get me wrong, if you're taking the stance where you won't install games with DRM, I tip my hat to you for sticking to your principles. Don't blame Valve for building a successful platform, but blame the publisher/developers for not picking multiple options. In the end, nobody is forcing the developers to use Steam. There are plenty of games that have both a Steam and a non-steam release. Go look on gog.com for the more recent releases and lookup the games there on Steam. Plenty of indie publishers choose both options as well.
So if I have a problem with Steam's EULA or ToS, I'm basically unable to play the extreme majority of top-tier titles, and only some of the indie titles out there.
Well, yes, that is how you should stick to your principles. You get over that fairly quickly though. I for one refuse to buy consoles, and I miss out on a lot of very nice console exclusives that I'd like to give a spin. But hey, look here, other games I can play,... There are more interesting games out there than I can play in my lifetime, especially if I take my other hobbies into consideration.
However, this in turn might reduce the motivation to make a DRM-free Linux (or Windows) games if Steam is there and us minority fellows aren't worth the trouble.
Publishers/developers are rarely going to release games without copy protection. The first few weeks after release are far too critical for their sales to go without trivial copy protection, and with the ease they can implement one of the DRM schemes these days they'd be foolish not to implement one, no matter which platform you're talking about. It's just the way it is, and even if Steam were to stop existing tomorrow there'll be another platform to fill that void before you can say "Oh wow, who'd've thought". DRM has become so much part of that industry that you'll either have to accept it or learn to do without those who implement it. The rare few cases where a publisher changes their mind are because they're getting terrible PR and fear they'll lose their sales, but with DRM being so widely present in games today you have to implement something nasty or be dealing with the wrong audience to even get that reaction.
it means DRM will never leave us because too many gamers cannot stand on principle, or simply don't care
You can raise awareness, but there are always going to be people who don't care. And to be honest, out of all the forms of DRM, Steam is the most widely accepted one, which in my opinion is because Valve all in all has a pretty decent reputation as a company and it's very convenient.
As for the whole gaming on linux thing, I would really like to see it happen, even if it comes with the DRM from Steam and what not. It would certainly be interesting, and more attention to Linux as an OS is in my opinion a good thing. In the best case it could make developers consider Linux as a viable platform for release, in the worst case you'd at least get the Valve catalog on Linux. No matter what Steam does, there's always room for developers and publishers to release their games on the platforms they choose without DRM. But that in the end depends on the demand for it, which all things considering isn't all that large.