Haha, I agree but it's funny you mention Kasumi because she is one of the many squad mates whom I found filler content. There was absolutely no reason to hire a thief to fight the Collectors, either before exploring their ship and learning their identity or after. Same with Thane, having both Jack and Samara as the biotic superstar. Really, Mordin is the only companion who has specific plot relevance. It would have been nice if there had been fewer companions with greater focus on each of the remaining ones, and that they would be more than tangential to the Reaper plot.
There wasn't a reason to hire a thief, but I'm always drawn towards the character who is a bit more lighthearted about things and just overall fun to be around. It's also why Alistair and Varric are two of my favourite companions in Dragon Age.
Mass Effect 2 left me with such a great feeling, I do not think that structurally, it was obligated to focus on the reapers directly. In the end, the release of Harbinger after we disarm the collector base re-enforced the malevolent and menacing threat of the reapers which had at the very end of Mass effect 1 still been perceived as rather removed and impotent because of Shepard's Citadel/Sovereign victory. And so Mass Effect 2 began with enough leeway to explore a route of internal character development, overall universe and lore enrichment, before kicking it up for direct reaper confrontation in Mass Effect 3. I would argue the reason Inquisition felt so weak was because it introduced a very strong antagonist but we faced him with an under-developed protagonist and team, alongside a lot of new lore, so the drama was not as intense. Whereas in Mass Effect 3, we have all this side story in Mass Effect 2 - which, mind you, was not entirely irrelevant to the reaper threat at hand.
Also, there is a difference between a mistake and an intention in authoring plot points. Just because someone goes from point A, to point B, to point C, does not mean point B was a mistake. Shepard and everyone in the Milky Way is, in the course of Mass Effect 2, learning about new developments with as little information as we have. We don't really know much about the reapers. We've only seen sovereign, seen one of his agents. In Inquisition, they blow up Corypheus in our faces on the get-go. There isn't a lot of mystery or removed foreboding. Corypheus shakes our hand at Haven. "Hi, I want to open up the veil, hello, take over the world, want to become a god. This is what I plan on doing. Bye." In Mass Effect 2, we are investigating the collectors. Literally, we have no clue what's going on. And that's why the Illusive Man sends us on all these missions to investigate. So it never truly feels like we are derailed. We don't even know the track we are being derailed from. It's simply going along a journey, as a story ought to be.
Well I think Mass Effect 2 had the best character development of the series but when talking strictly about the main plot, I think there are a few issues to be had there and that it's not the perfect story other people tend to make it out to be. Not to say that it was bad or anything, it was still an extremely well done story and I like the whole "build up a team to take on the enemy" thing it had going on.
Inquisition on the other hand had a weak main plot but remained with strong character development. I also happen to think Shep is kind of boring as a standalone character not counting headcanon stuff, so I don't really see it as a plus over the Inquisitor who is the same way, My Inquisitor left more things open to actually be able to roleplay than Mass Effect did for Shep.
To clarify, Mass Effect 2 as a standalone story I think works largely great but as a piece of the trilogy it does nothing. We learn very little new about the Reapers themselves, and we do virtually nothing to prepare the galaxy for the upcoming game where we fight the Reapers. Everything is about the Collectors which are an isolated threat contained entirely within ME2. Other than Harbinger making meme worthy taunts at us, the Reapers weren't even really in Mass Effect 2.
Corypheus' problem was that he had a fairly strong opening and got to deliver his big evil villain speech before we were forced to destroy our own base just to get away from him, but after that we proceeded to curb stomp him at every turn. If you're going to introduce a villain that early on, I feel that he should remain a threat and not just a punching bag for the protagonist, which is ultimately what happened to him.