"Ah, you know it will.
Samara- job is done, so she is no longer under any obligation to Shepard.
Miranda/Jacob- Could well be hunted by Cerberus (Shep only gets off the hook because s/he is still the main hope for Reaper destruction) and have to run off.
Thane- Too sick/dies.
Those are the only four I can cover at the moment, I'm too dizzy to go into everyone x.x I can only see Tali and Archangel sticking on, simply for the sake of familiarity. All that said, if I am right, I will forgive Bioware as long as they include a hanar squadmate and/or keep Legion around."
Mordin- gets to old and dies/or becomes a support character like joker or chakwas. Seriously what's up with their lifespan?
if all the worlds are under attack at the opening any crew member could threaten to leave. "The turrians need us shepard we have to help them!" "No we should meet up with the migrant fleet, from their we can launch a counter attack" "This unit does not have enough information to make an informed decision"
I think shepard should have to pass a speech check to make a decision/not lose people. people who have played the old games start off with enough clought to actually choose their route. My first moves would bring the quarians and geth into the fray (seems like shepard would be able to)then we rescue the turians and earth for their fleets. cure the krogen genophage, find the rachni, and continue to wipe the reapers out/rescue people and maybe we have a chance. How many times did the reapers find united forces to wipe out? Still kinda seems like an uphill battle, but hey that's what makes it epic.
P.S. Oh and I thought it was kinda weird that you thought Sumara would leave. Shepard "well I'm off to save the universe" Sumara "sounds like a worthy and honerable goal...see ya, nice knowing you." Shep "WTF?"