The simple fact of the matter is, as far as getting squadmates killed, is that unless you're intentionally trying to get people killed, or you skipped a bunch of loyalty missions/upgrades, or were a complete idiot about choosing who does what in the suicide mission, there is no way you'd have more than like 3 or 4 people die. First time I played through the game with no guide, I lost grunt as the distraction team leader because I figured "well what could be more distracting than a big angry krogan", and that was it. Next time around I picked a leader person, got the no one left behind achievement, and it was easy.
Even if in addition to picking the wrong distraction team leader people got fooled into Miranda's any biotic could handle it idea on the shield part or took Jacob up on the volunteer option, and then took grunt and garrus to the terminator without ever downloading Zaeed....that's only 4 deaths with everybody loyal.
Ruling out the DLC characters, how many people have you got left? 6. Which was the size of the party in the first game.
You would not be able to get enough people killed to have a party too small to be viable for ME3 unless you were being hasty and reckless with your choices, or you were trying on purpose to get squaddies killed. And I don't see anything wrong with punishing decisions to not bother preparing for the suicide mission or get squadmates killed on purpose. People always complain the choices don't have enough consequences, well here's a consequence right here that you can have.