Maybe those would actually be advantages in a world where network delay and connectivity issues aren't a thing.
Stadia reminds me of the time Homer Simpson designed a car. Just because an idea sounds ingenious doesn't mean it will result in an ingenious product. Actually, Stadia is a lot like Homer's car. Instead of Homer, the reigns were seemingly handed to engineer-types living in the Silicon Valley bubble who thought it'd be cool to play games over VNC (an oversimplification, yes). The product should never have seen the light of day and, when unveiled, everyone who saw it knew it was doomed.
In reality, most people just don't care about downloading game updates, regardless of how unreasonably gargantuan they can be. It's arguably a good thing that people have to take some break from gaming. Regardless, there's no evidence that downloading updates is preventing any meaningful number of users from spending more money on gaming. Anti-cheat software sucks, but again, it's not much of a roadblock for most gamers if at all.
The disadvantages outweigh the benefits. Input lag is frustrating, even if it happens occasionally, and is borderline unacceptable when playing online multiplayer games. Lose access to internet? Just go read a book, kid!
At best, Stadia might have been a niche product for the minority who would want it. Not in a million years would I have wanted a Stadia.
To compete in the highly competitive sphere of gaming, Stadia needed more of a value proposition than being able to play games instantly without downloading updates.