Thanks for the discussion; it made me change my mind. Regarding performance, I think we can make optimisations during the alpha and beta phase, if the PR is merged and if performance optimisations are needed.
Thanks for your input, everyone; highly appreciated!
Well, the poll is now closed and it came out 17 to 4 in favour of dbm.sqlite3 being the new default. Interesting! 21 votes is a decent turnout, but not an overwhelming one; dbm is perhaps not the hottest topic 
(My PR does not make dbm.sqlite3 the new default; there is plenty of time to make it so before the feature freeze, if we want it that way.)
Shouldnât we make it the default now so it can be tested in the alphas? We can undo the default portion if it turns out to be a problem.
In my defense, I will point out that I voted before reading the discussion. It was just a shot from the hip. 
So, feel free to make it 16 to 5.
I think that is worth considering. For the open PR, I suggest to leave it as it is, and instead focus on the feature itself; we can amend the dbm._names list in a separate one-liner PR combined with a more focused discussion.
FTR, the feature PR was merged and a follow-up PR for making dbm.sqlite3 the new default (at least for the alpha period, as suggested by @stoneleaf) is scheduled for merging.