uhh, it's a game streaming service, why the fuck would you buy your games as a game streaming service if you wanted offline access? nobody's forcing you to use Stadia or a similar service...
It's one thing for an offline device to have online only like an Xbox, and getting a game version that specifically is online only when an offline version is available. The games-installed-locally crowd isn't going to disappear anytime soon, especially since offering an online download (local install) version alongside a streamed version incurrs close to zero extra cost.
The only possible "doomsday" scenario i can imagine is something like consoles appearing but for the streaming end. A few kinds of custom built console-like computer cluster for simultaneous play appearing on the market, and them becoming so prevalent on the server end of streaming that all the major players in the industry just optimize for those systems and not home computers, so that the cost of supporting local installations becomes enough of a problem to cut support for it entirely, or similarly, game devs only focusing on supporting games for the few big streaming service providers, but in that case, i think the impossibility of anyone else entering the industry (an impenetrable oligopoly) would be a much bigger issue than you not being able to install Far Cry 25 on your PCI-e Gen7 SSD