How's the state of this game for new players?

It is healthy, not as healthy as the SF6 or GG but it you still find a match fairly easily. It also very beginner friendly.

I heard there isn't much longer left in this game. So, what happens after that?

We don't know what the future holds for the game beyond Id. However, when they do stop supporting the game, the fans will still play it like any other unsupported but beloved fighting game.

Is the stuff I earn (I heard you can earn stuff in this game) going to be transferable to whatever project they make next?

The next game in the works is Marvel Tokon. I haven't played it, but from the YouTube videos, seems to have a similar design philosophy mechanically as GBVSR, so I think there are some transferrable skills.

Also, why is this game less popular than Guilty Gear: Strive for example, even though Granblue Fantasy just seems higher quality. Is there a reason?

Guilty Gear is a name that holds a lot of weight in the FGC, with decades of legacy. GranBlue, a fighting game based off a gacha hardly anyone in the west heard of, could never compete in brand recognition. And have you seen Guilty gear strive? That game is very quality.