It makes no SENSE for certain characters who survived the suicide mission to abandon you, especially with their knowledge of the upcoming Reaper and Collector threats.
It didn't in ME1 either, but then Bioware ASSUMED DIRECT CONTROL and forced the plot to dance to the arbitrary tune it was playing by killing Shepard off and fast forwarding two years.
Garrus confesses Shepard's practically his only real friend, Tali's name is "vas Normandy" and she could be exiled with no other home, Samara swore an oath to follow you, etc.
Right, and they're extremely popular characters who will probably return. Mordin might die from old age, Legion might finish building a consensus and decide that he's needed elsewhere, Miri's dad might catch up with her, Thane might not get cured and also be dead, Wrex might decide that he needs grunt as a PR thing for Clan Urdnot, Zaeed and Kasumi's contracts have expired, and so on.
****, maybe the SR2 docks in the Citadel and C-Sec decides to compound it and arrests your crew for being Cerberus operatives.
If Bioware wants something done, it will be done.Probably not. Will there be some new characters? Very likely. But at this point, it would be very disconcerting to spend all of ME1 and ME2 with Garrus and Tali by my side only to have them sit out the final hurrah for some arbitrarily contrived reason.
The only crew members from ME1 that didn't have a contrived excuse for not rejoining you are the one that died and Wrex (either for also being dead, or for being leader of Clan Urdnot). I don't doubt that every single one of the surviving characters will
make an appearance in ME3, but squadmember status is unlikely for all but the most popular characters (Legion, Mordin, etc).