Their success depends on several factors.  The most important one is "are they willing to stick with it?"  The tech they developed is actually pretty impressive, but that isn't what leads to success in gaming.  Are they going to invest the time and money needed to learn what it takes to succeed in gaming?  Microsoft actually had some experience developing PC games before XBox was a thing, but it still look them a generation before they really could be a serious player in the console space.  It takes time to learn the gaming business.  Games are actually a lot more important than the tech.  Google may not have even realized that yet.  They need to do some serious investing into making first party killer apps.

A second major factor is that I don't think their main competitors are the big 3 console makers.  I think their main competitor is Steam.  Steam is most successful wherever consoles don't have much of a presence.  What do potential gamers do in countries like India or Russia?  They game on Steam.  I'm sure that there are a lot of people around the world that feel PC gaming is too expensive for them, but they can find a way to game on their phone.  That is where the potential success of Stadia is: in people who want to be gamers but don't have the money or access to games like people do in North America, Japan or much of Europe.

The third factor to keep in mind is that we don't really know how much of a success/failure Stadia is and we will probably never know.  Where is the data?  We don't get data from Google or from Valve or Microsoft or Sony or any of the major third party developers.  How can we track it's success?  We can't.  Unless it becomes a major cultural phenomenon in your country, you'll never know how successful it is.  If it does become successful, then it's success might sneak up on us.

Bonus: I really don't like the direction that digital distribution is going, but this has very little to do with Stadia.  The big 3 are steadily moving toward digital distribution with Nintendo being the last potential hold out.  As long as Nintendo keeps making physical games, then I think all of the big 3 will.  A large chunk of the market really wants physical games.  On the other hand digital distribution is kind of a Prisoner's Dilemma situation.  It's easy for the big 3 to keep heading in that direction, but if these platforms end up 100% digital then they will all be wose off (and we will be worse off too).

I think the biggest thing holding cloud gaming back is that the main target audience doesn't know it exists or doesn't realize that they want it.

Don`t think the want to be gamer that can`t buy a console or PC would have good internet and willing to put 60USD on a game.

You are assuming that they aren't willing to adapt to make the service work.  A company has to be willing to adapt whenever they develop a new business model.  That is true for any new business model out there.  But if they don't adapt or if they give up early, then yeah they are sunk.

If they stick it out a while and are willing to adapt, then there is a huge market out there.

And they will be offering the AAA game of third parties on their service for a lot less than those companies will have on the consoles?

AAA games do not stay $60 for long.  Also, as I said I don't think Stadia can really compete with consoles, but I do think it can compete with Steam.

That is true, but are you thinking on the game releasing on Stadia 6 months after other platforms, faster pricecuts or another way?

I haven't paid too much attention to it. I figured I would circle back to it years later if it ended up having some good games. Gylt looked pretty good, hopefully it comes to other consoles if Stadia crashes and burns.

Nope.  I'm thinking of it selling to people in places where there aren't many consoles.  They have a phone and a controller, so they can play GTA5 or Minecraft for the first time.  Games like that.

Depends on your point of view; It is a non-starter, but on the hand you could also vote that your view on Stadia’s success is “Good”, if you want it to die as soon as possible. Like me.