I think people who like all three games can be "true" ME fans, plain and simple, because the lore has been inconsistent-yet-adaptable from the get-go.
I also think chalking up ME2 as a successful entry and ME3 as a failed entry simply isn't sound reasoning, because they're both "guilty".
Disagree. I have three reasons:
1) Nothing in ME2 came close to "ME3's" ending.
2) ME2 leaves you with an overwhelming feeling of success and achievement. "ME3" leaves you with an overwhelming feeling of misery. I know this is subjective, but then again this is mostly what I mean when I express my general lack of appreciation for people who liked the endings or just thought they were okay. They were a kick in the nuts.
3) Even the ending and its emotional aspect aside, there are more inconsistencies in "ME3" than in ME2.
Points 1 and 2 have some level of overlap. I'll elaborate on point 3:
Actually, I'm not sure inconsistencies is the right word when it comes to ME2. ME2 surely has some very stupid crap in it if you think about it, like the Human Reaper, and it also railroads the player in a way people may have problems with (Cerberus debacle), but as for inconsistencies, as if, things in it that are very different from how they were portrayed in ME1, there's the nature of Cerberus (Alliance rogue op group -> terrorist organization), thermal clips - but that was a gameplay decision, and, what else... Joker's Vrolik syndrome mysteriously spreading to his whole body, probably.
But apart from that, there was also much care put into ME2 and attention to detail. This is getting lengthy, so I'll cite one example - if you look at the uniforms of scientists and soldiers who appear in holograms on Pragia, in the facility where Jack was held, they are exactly the same as the ones Cerberus used in ME1. That's attention to detail.
More to the point, in ME2 they actually tried to cover up their retcons. I know that saying all the Cerberus teams encountered in ME1 were splinter groups and that core Cerberus isn't that bad is very thin. But at least the devs tried. They saw these inconsistencies and acknowledged them, for better or for worse.
At the same time, in "ME3" you get an entire race's way of thinking retconned merciessly without even a bit of explanation (geth). You get people so out of character that they aren't themselves any more. You get hair-brained decisions like killing off a fan favourite episodic character via Twitter. And all that is handwaived, there isn't even a slightest attempt to cover up retcons, which are much, much more numerous and destructive than any of the problems ME2 had.
And finally, ME2 feels polished, cared for, complete, as it is. "ME3" feels like a rushed mess. Which it was.