Most digital gaming stores are, except GOG and ItchIO. Even consoles are trying to push things that way. XBox has Game Pass and Playstation released a version of their console with no disc reader. Subscriptions may seem more fleeting that digital purchases but in actuality we've seen how companies can take down purchased media and entire digital storefronts.
I have purchased more Steam games than it would be sensible but as companies lose any qualm to take purchases away from customers, if anyone wants any any guarantee of ownership they really need to buy DRM-free and back them up independently.
Games using Steam's DRM, have the benefit that if Steam ever goes down, there would be a massive amount of people interested in breaking it to free all the games at once.
It actually happens all the time, but Steam can roll out new "patched" versions of the DRM as long as it stays in business.
They are also aware of this, and even have promised to release a DRM bypass if they're ever about to close shop... but in practice it wouldn't really matter; whatever last version of the DRM they ever release, will get broken in record time.
I think more likely than Valve going under is Valve getting bought or going public. Both would result in the new owner (a megacorp in their own right, or greedy shareholders, respectively) turning the system into shit to squeeze more money out of it. And new DRM would be foisted onto the system regardless.
That's a possibility. Then again, Steam games are getting stripped of DRM right now (and possibly enhanced with some malware), so the moment the value proposition of just installing Steam and not having to do anything else goes down, it's likely for generic DRM strippers to appear, at least for older versions.