I also enjoyed Google+ despite Google's attempts to make it worse at the end. Still miss the discussions there. A few of the communities and people moved to forums or similar, but several I've lost contact with.
A good friend of mine had built his business around the Google Earth API. He made simulators for employee training as well as games.
Another good friend still relies on Picasa, as we haven't found anything that fits his image managing needs in an equally easy way. Sure it still works, but for how long who knows.
So if you depended on it, you could depend on something else. And if it was successful, it wasn't canceled.
I didn't work on Stadia but I imagine that it was a huge waste of money for the company for the relatively few users it had. If they could have offered a good subscription based service like Xbox Game Pass or Apple Arcade I'm sure it would have more users. But getting access to content would be very expensive. At least they refunded consumers' money for the games they purchased.
As to whether it was successful - I mean, it didn't exist long enough to answer that question.
Other things that come to mind are Chromecast Audio and OnHub.
The list is much much longer, I'm surprised you're having a hard time finding them.
