This kind of stance can be useful to a community, though, that wants to ensure future proprietary development remains possible (kind of strange to want but it can make sense). I recall when Trolltech did this with Qt. If they went out of business or there were a change of control, Qt would switch from GPL to BSD licensed.
One of the key business differences between RethinkDB and MongoDB is that the latter keeps some enterprise-grade features/tools closed-source and available under a commercial license, i.e. the "open core" business model.
That is why it is really important to have only the best and smartest VCs on board. Edit: Don't give equity up in your company unless your vision aligns with your investors and they can do more than just fund you.
This allows the company to get larger accounts with no real downside as they would be out of business.
I think continuation of development and community growth even after they closed down is what gives people confidence - and there's no practical way to show that upfront (assuming that having a big/visible community outside of your core company is not viable for such project in that ammount of time) - re-licencing just the first step to that. They proved what happens in a "worse case scenario" but I don't think you could have done it without going trough it.