"I guess we can agree based on the outcome that it turned out to be one?"
Agreed. My perception is more that it is turning out to be one and that is a good thing.
"But I do know that in the end, MySQL was open where it really mattered."
The GPL means that there's no real fear that licenses will be scarce. And the large user community means that there will always be people to offer basic support (free or paid) and consulting. To that extent, I agree.
As far as getting bugs fixed, adapting to new use cases and changes in the market, or just moving the product further, the jury is still out. Will Oracle do it? Can any of the forks do it?
Or the larger question: are you OK with calling MySQL, as a product (i.e. the engine), "done"? I'm not saying that in a negative way. We rarely talk about software that's done, or what it would take to finish a given product, or how much sense being "done" even makes.
"It seems in most cases you simply have a core group of developers doing their thing, with occasional discussions and input from the outside."
That's not how I would describe postgresql. There are lots of people that only occasionally submit patches and lots of people that take part in design discussions even if they never submit much code. It's pretty easy to start out as an application developer and then be sucked into a design discussion, and before you know it you are criticizing a design because it doesn't fit your needs.
Even if nobody has ever seen you before, if you jump into a design discussion and have a legitimate concern or use case that hasn't been considered then it will be taken as seriously as any developer.
"What I care about is software quality and proper documentation, those are not necessarily a function of community."
Absolutely correct. Although those things are also not a function of being open source or free of charge.