The flaw in their thinking (and the reason why they ditched 25 mans) is that whenever you queue up with a team for a premade, it requires each player to accept when the queue pops. Their words. So if one would be afk cause his wife is mad at him, the whole rated bg would fail and no one would be able to enter. Same thing applies to 15, though it's perhaps easier to manage that.

The most reasonable solution for me would be that when you queue up with 10 players, it takes 8 confirmations to start the bg, and the 2 can still join (teleport) when the bg is ongoing. If you have too many people who are always late, just boot them for the next time you form a premade. I.e. 80% (or 75%, whatever number will work) should accept the queue in order to start the bg.