Daniel @ RethinkDB here.
As you mention, RethinkDB is fully open source so RethinkDB is always going to remain freely available.
The cases in which the community forks a project licensed with a copyleft license (like LibreOffice) has to do with insatisfaction with the direction in which the company that owns the original trademarks is leading said project. There's no risk of closing the source code.
Slava @ RethinkDB here.
This has one exception -- the copyright owner can choose to start releasing enhancements as closed source, and they wouldn't be legally obligated to open source them. Currently RethinkDB, Inc. owns the rights to the code -- if we wanted to continue developing closed-source enhancements, technically we could. If we were acquired and our acquirer chose to do that, they could too (since they'd end up owning the rights to the code). Copyright and licensing are different things -- essentially the copyright owner has the right to relicense future code in a different way.
We have extremely strong incentives not to engage in bad behavior (and it runs against our beliefs), but I thought I'd point out that there is no legal barrier.
This is also why many open source startups don't accept code submissions from outsiders until they've determined they're going down the path of a consulting-focused business model. It's just too risky.
- You & and your team have put in 5 years of effort. - You have generously shared your mind product. The community has it.
I don't [know] about you, but over here in NYC, we have to pay for food, rent, clothing, medical insurance, entertainment, etc. Not a single one of these transactions involves counter parties that would smile my way and say "it's on the house".
I'm not suggesting that is likely, but that it's more what would/could happen -- there is already a real, working, full product that wouldn't disappear over night. And a commercial fork would likely be pretty painful for everyone, just as closed Solaris is looking less and less interesting as both many of the minds behind the great parts of Solaris work on Open Solaris in one form or another, and as fewer community resources go into closed Solaris.
I see the AGPL (as opposed to some ad-hoc license) as another benefit for RethinkDB. Many might not like the copyleft-part -- but at least it is a known and well-documented quantity -- no surprises likely to come from/with forking and/or when trying to merge with other Free software (be that BSD or GPL or...).
(BTW the Open Source Institute was unable to get a trademark for "open source" so it doesn't matter that they approved it.)
Now, one can be in agreement with the idea that the four freedoms are important, especially as we increasingly live in a world where software is not only convenient, but necessary in our daily lives -- but the idea with the AGPL, and why it is needed for server software -- is pretty clear.