Has Google announced any policies these topics? It's like you have formed the mindset that will just allow all this.
Most of this subjects are not really tied to Google's policies, but to the general behaviour of the market.
-Always online DRM is necessary on a streaming only system, with all of the negatives that implies.
-Competition with older titles has been something rather frown upon by big developers. How many servers have EA or Activision closed down, effectively killing many games? The difference is that, because you don't really own or have the game in any capacity on a streaming service, you can't even revive it through homebrew methodes. Your purchase is basically moot at this point. You get nothing.
-Game preservation will be moot as well, so if they want to censor a game (or erase it completely for whatever reason) they can do it and there's no possible way to stop it. At least with physical media (and digital to some extent) you can keep them on your hardware until they fail. Hell, as bad as piracy may be in some cases, it is the only acces to a plethora of abandoned games nowadays, that is a fact.
-Google is an advertisement-based company, so suspicions about intrusive ads is not unfounded. There's also the possible problem of Google tryint to sanitize the content kept in Stadia just like they did with YouTube, potentially affecting the artistic freedom of many developers.
-No access to the game code means no mods, so that is just out of the question.
-Account stealing could make one's purchases completely obsolete. On a regular platform, you at least can access the single player games you have installed (as long as you don't need always online DRM). No luck with Stalia, because it is always online and you have nothing on your hardware, so someone can steal your accound and you are left with nothing until the problem is resolved. I forsee this being a massive plague during the first year of the Stalia's lifespan.
You know it deserves the GOTY.