Palm was a dominant mobile platform until Palm nuked itself with 5 years of clownade with Palm OS 6
Symbian was the biggest, most dominant mobile OS globally until Nokia didn't decide to simply trow it away.
JM2E was the most largely adapted app platform, even into early Android years, until Oracle/Sun simply stopped licensing it...
Now think just how thin is the ice Google is on now with it flirting with idea of "rebooting" Android with Fucsia
I'm not familiar with Palm OS, but Symbian and JM2E both has the iPhone and Android OSs as growing threats. Google hasn't officially stated that it is rebooting Android, and even if they did (and did it right), there isn't a major alternative lurking around the corner...
They have experience on chat and payment software and infrastructure system used by millions of people in their country.
The Chinese market on the other hand will always have users, is huge and I presume most developers won't be willing to give up on it. If they do, someone else will serve their customers and might become a future competitor globally.
China-focused apps will only have to re-implement google services. That sound a lot easier to me than supporting an entirely different OS. If those versions can run on google phones as well, devs have the option to drop the play store version down the line to save cost.
I wouldn't bet a lot on it, but this might be the best chance we ever had to get a real play store competitor.