Can Gamers really be boiled down to such a homogeneous group though? I think many of the younger fortnite console gamers already have very different views from your average young/middle aged adult on steam.

Stadia doesn't have to steal all of steam's userbase to be able to reach critical mass for their own userbase and videogame streaming seems like a logical next step (like music and video streaming were) if a good UX can be provided (and I think they're aware of latency concerns and would not enter this market if they thought it unfeasible to provide a good gaming experience).

Disclaimer: I work for Google but am otherwise unaffiliated with the Stadia project