I wonder if they could have closed more deals if they'd promised to do something like this. Then again, maybe the board would have balked at that as a sign of no confidence.
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.
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.
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.
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.
This allows the company to get larger accounts with no real downside as they would be out of business.