I don't for a moment believe that only Casey could have made the games, as the writing is probably the strongest point anyway (the part that requires the least directing, and a part that he gets no credit for).
The pacing in all three games has always been a little iffy, though tolerable. Well, until the end of ME3. The rumours tell us that Mac Waltersr and Casey did this on their own, and going by the drop in quality, I'd say this was entirely possible.
If Casey was a sports manager and his team had done something so badly in the last ten minutes that they went from a strong victory to an all-out defeat, we'd question tactics, but for the most part blame it on a lack of focus from the players. The issue here is that in the case of the last ten minutes of ME, we're hearing that the players had no input. They weren't out there on the field finishing off their strong game. Instead, they were all brought onto the bench for a breather with just the manager and one player out on the pitch, and the end result is painful. It doesn't matter how good they are, a two-man team is not going to work.
Any game that requires a team, be it sports or electronic, only works well when you remember that you're all in it together. If you take most of the people out of the loop, only bad things will happen.
I'm not saying Casey or Mac Walters should take the blame, as such, but rather 'take one for the team'. Even if they aren't responsible for the mess, someone high up should stand up and say 'yes, this was my fault'.
The simple fact is that the ending for a lot of people was ruinous, nonsensical, both, or just underwhelming. Of course there are people who like it, and that's fine, too. But it doesn't make much sense to shout about the people who loved it when there're enough people saying that they don't agree.
The Extended Cut DLC would have been the perfect announcement to use as an acceptance that people didn't like the ending, but instead of addressing people with concerns, they instead made an announcement that implies that the ending needed to be made simpler for a majority of players.
It doesn't matter how you coat those words: Implying people didn't get your ending, and need clarification is just as bad as flat-out stating they're too dumb to understand.
Anyway: No, I don't think Casey should be sacked, but I do think that whoever is responsible for the ending should come forward and acknowledge it. I don't even know that I particularly want an apology, as it'll either feel hollow or forced. I would like to see them prove they were and are listening, however.
If they are monitoring threads right now: Fix tali's face!