The positive effect on balance and speed would be more important than the reduction of flexibility.
I would rather wait a couple more minutes for the matchmaker to fill the empty spots in a lobby that I've been enjoying than be torn away from players I'm having fun with.
Numbers DO NOT adequately reflect QUALITATIVE player experience. Being shuffled around to new lobbies is not fun. Being forced away from a fun match is not fun. Being stuck in crew formation watching a spinning circle is not as fun as chilling in a lobby with ships of my friends to talk to.
Qualitative player experience seems to be lower on your priority list than your quantitative statistics on MMR and balance. Not a fan.
Maybe you could have it base rematches remaining on how many people leave afterwards. If nearly everyone is sticking around, don't make them leave; if enough people are leaving that it's burdensome for the matchmaker to fill it, have it go to crew formation. I don't know. Something, ANYTHING, to make it so that if WE ACTUALLY HAVE A GREAT LOBBY that we don't have to have it taken away because sometimes unbalanced lobbies get rematched.
"sometimes unbalanced lobbies vote for rematch" and "sometimes a bunch of people leave a lobby" isn't a justification for "you can actually literally never keep a lobby going even if it's enjoyable for most people involved"
Maybe we could have an option at the end of the match where you could vote "rematch", "return to crew formation", or "I individually want to leave and find a new lobby" so that the matchmaker could know in advance how many people are leaving and how to treat the lobby?