Not every game on Steam has DRM. There are plenty of indie games on there where you can download it, copy the files away and play them offline. And for what it’s worth: Gabe Newell did promise that if Steam ever shuts down they’ll offer you all your games as downloads (though so far they have been solid).
I’ve never heard of a legitimate Steam account getting banned. You might be thinking of VAC bans (where your account is flagged as a cheater), which can force you out of multiplayer servers. But losing your entire Steam library? Unheard of so far if you don’t mess up on purpose (like trading stolen items, excessive account sharing, using a stolen credit card, doing chargebacks, etc.).