the intro to the Octotree Github thread basically sums up open source perfectly!
a) You need to have absolute 100% of the code ownership/copyright in order to change the project license. This is not true and there have been precedents where a valid and successful relicense was done with much less (if I remember correctly I learnt this from a comment somewhere hidden in that same Github issue)
b) The other project contributors would not necessarily agree with the relicense. Which was not the case: the most prominent contributors showed up and explicitly gave the author ownership (or free license) over their small parts of the code.
I suppose you could have somebody quite ideologically attached to the GNU/GPL who could then hold up a change. If the code was core and you couldn't easily re-implement, could make the price high.
At the time, the failure modes of the more permissive licensing model (for example, of NetApp's innovations for things like WAFL were never contributed back to BSD) were more foremost in our mind, and so for us the GPLv2 was the right compromise between the rights of the community and the companies using, and improving, the code. Of course, the decision that we made in 1992 was in a world very different from 2020, and so hopefully the choice of the Apache license will be the best one for TerminusDB. I have only the best wishes for you and your community!