I kind of understand the trolling argument, but
1. You don't want to play with those people anyway.
2. Captains can already do this, and noone ever does, except to reasonably grant themselves more time to deal with crew loadouts, most often from, again, this exact problem, people joining last second.
Since we don't allow kicking etc, because we trust the community enough, this seems like a good use of that trust as well.
I am 99% confident this will NEVER be used for harm, at least persistently.
I'm also okay with the "this game is about to start" lockout, this is a good solution as well.