I asked this years ago elsewhere, and found a answer somewhere.
APPARENTLY, BACK THEN, Gabe said that in that event steam would release the...dlls? appls? Something-ls, offline play lite basically. I think it also means you HAVE to have the games downloaded - purchased but not downloaded would be a no go or a rush-grab to DL everything....
There are many concerns however - whether they WOULD do this, because that was YEARS ago, before GaAS and all that, and what will DRM do to your games, the like. Just because it might be both easy to do and the 'right' thing to do does not mean that Steam, the Publishers, or both WILL do it. Gotta get used to being forked over, cha know?
That reply from support doesn't raise any hope. It's too haughty. Too much hubris. Especially since Steam is asked this question every odd week, they should have a cookie-cutter response from years ago.
And it's going to happen, eventually. Steam won't last forever. Nothing won't. Best thing we can hope for is that it lasts longer than we do, and if it doesn't, that somehow, they do the right thing. Because honestly the industry seems to be gearing up to never do the right thing ever again. Hell a recent article says they might have to be sued into actually doing anything.
It's part of the reason I went over to GoG and try to buy games there first than on Steam. No, really. I like just having the .exe file stored somewhere.