I know ME2 is regarded as the transcendent all-time game of the series
Really?! By whom?
Worst of all, the game had no plot continuity and was like a complete diversion or side story that just ends up back where ME1 ended to setup the actual conclusion of that story in ME3, minus crucial development that could have made that conclusion most effective. It's otherwise personality driven with the recruitable characters, which people obviously liked, but I was expecting a lot more than that, and most people were probably just staring at Miranda's ass.
Where they added insult to injury is that all those new characters become NPCs in Mass Effect 3. A few are given a little "last mission" to conclude their arc, but most only get a couple optional lines of dialogue. So basically all that recruiting that took up a whole game was useless, as the crew for the actual final mission in ME3 is either composed of original ME1 characters (Liara, Tali, Garrus, Kaidan/Ashley) or newly introduced ones.
Furthermore, the characters in ME2 were inserted to the detriment of Liara and Wrex, who were in my opinion the two best sidekicks from ME1 (who cares about Kaidan and Ashley). They chose to make Wrex barely an NPC due to the choice at the end of ME1 which I honestly can't forgive. Instead they brought on Grunt, the most vanilla Krogan possible, which was pathetic. I don't care about their excuses, they should have worked around their plot to have Wrex as an alternative to Grunt or something. It could be done, they just didn't care enough. As for Liara, they basically assassinated her character and no amount of shitty comic books and "I'm so sorry" DLCs really fixed it for me.
And like Griff said above, it's easy to just blame ME3 for it, but it was a given that they wouldn't have 15 sidekicks in ME3 if they had to compromise like they did in ME2. They planned badly because they had no idea where to take their story after ME1's perfect ending. ME2 should have done away with the "evil Shepard rebuilt by Cerberus" bullshit and instead have him struggle to rally non-allied races like the Batarians against the Reapers, uncover and undo a secret plot by Cerberus (maybe tie-in the Collectors if you really want to), find some secret Prothean tech that could help in the last game, and recruit two or three squadmates in addition to retaining the original ones. Then at the end, have the player make a choice where some sacrifice themselves in a real suicide mission that makes a difference and has an actual emotional impact versus "well, I better reload".