If Steam ever shuts down, an old Reddit post had info about a customer experience message chain where a Valve representive said they had contingency plans if that ever happens.
If there's third-party DRM on top of the regular Steamworks DRM, the game you bought has much worse odds of actually working offline if Valve goes out of business or the devs abandon it. That's why I try to avoid Denuvo.
That being said, I've played Denuvo games on my system and any performance drops have been minimal at best. YMMV on that, but I'm a mid-end gamer at best. A certain athetic woman will provide alternative access to the game if you really want to skip Denuvo, but it should be fine.
But Denuvo is just silly and pointless. It sets out to be an anti-piracy measure, but it's not actually preventing games from being cracked at all. Game studios should prevent piracy by making a high-quality, consumer-friendly product. I would never pirate a GOG game, but I can't say the same for Ubisoft.