Prepare for wall'o'text
Well, I've got to admit that I just flew over the thread, sticking mostly to the posts of Bluko.
I'm sorry if I will repeat previous assumptions, but I think it's not healthy to read 56 pages of a topic that is bound to have some flaming in it

.
But to the topic:
Bluko, you make some really logic assumptions, taking the effort BioWare would have to integrate that number of characters and possibilities, if all old chars would come back as squad-mates.
One of the biggest arguments for your hypothesis is Conrad Verner.
It might be a detail, but regarding your words of pessimism, I'm assuming this could be the tip of an iceberg.
Even if you where paragon in ME1, he insists on having a gun shoved in his face.
BioWare did a very lousy job there, only implementing a feature to say it's there and a imported save would have some more impressions on the game than higher politics you barely encounter ingame or the reactions of some NPCs.
If it would have been done right, we would have encountered Conrad for example as a tourist who had very bad luck in booking a trip if we choose the paragon-way in ME1.
Now that BioWare is a studio of EA, we have to assume even more errors in story-telling since, even if BioWare want's to make the content, a EA-manager sits in their back and forces them to change plans.
Don't get me wrong, some games EA makes are funny, but if you look at what has become of Command and Conquer, it's possible to assume the worst.
I don't want to minder the decisions that take over in our Savegames, but I expected more of them.
Feros has for example only one sidequest with either the asari you let live or some other character asking for your help.
Noveria had one side-mission too and the rachni-queen may turn up big in ME3, I'm still somewhat optimistic about that.
After stating my pessimistic view with examples you're free to pick apart for further discussion(no criticism intended

), I'd like to criticize your idea in one specific point:
Shepard can die too.
Although it's unlikely, that BioWare will create several new characters just for replacing the ones that died in ME2, it's also unlikely that the will create two new Main-Chars we would play while still putting all the effort into mshep/femshep.
Taking your argument, it would mean we would play a whole set of different new characters, maybe even other species.
But since the ME2-game can only be imported into ME3 if Shepard survives(at least one of the loading-screens states that), ME3 will likely assume that Shepard survived.
So new players will have a new Shepard and "Veterans" their old Shepard.
Given the assumption that BioWare won't transform Shepard into a reaper-slave to create a new character after him/her or any other decision that somehow weakens the position of our beloved hero/badass-Shepard which would ultimately end up in a flamewar that is likely to kill the BioWare-personnel by overheating the computers of the community-department until they reach meltdown.
BioWare, I like your work so far, so I suggest you buy some escape-pods if you go in that direction

.
In the end not only the question about the squad matters, but also for the story in ME3.
It's pretty obvious, that we have to convince the council, volus, elcor, hanharr, the terminus-system, the batarians and every other organic species to prepare for the fight with the reapers, ultimately ending in a epic space-fight or viral attack against the reapers.
Possibly a mixture of both.
Also we should keep in mind, that BioWare is still a studio that want's to make cash, so the cameo-theory isn't that far away.
But then again, it's the final of a epic trilogy(if the income is right, the franchise is bound to be expanded), so treating the costumers by implementing the characters we all came to like, is quite a factor to keep in mind.
So I guess, the following theory is quite adequate, given the fact that ME3 has to assume some things(import surviving shepard only, two squadmates alive, etc) there will be a phase of the game, where "importers" are treated better(after all, this encourages new costumers to buy the prequels).
Basically they will have their old squad-mates for a short episode of the game, dead chars are replaced by "Cerberus-Agent" 1 to X.
When the game hits the main-part of convincing the other species, the old squadmates will leave, doing their best to convince the people(Garrus for example C-Sec+Turians or Terminus, not sure about that).
When they leave, the people who have to be convinced will send someone with you to validate your findings.
But if the Garrus, Tali, etc. where dead, you just get that new guy you have to convince.
And at the end, our beloved old friends will sit at the spearhead of different armadas or they are replaced by random characters if we choose to let them die.