They could have also left it open and still moved on. Good ending, open to more, but say "We are done with this now, byeeeee". Less hurt all around. Makes no sense what they did.
A thousand times this.
I can respect them wanting to move on to bigger, better, and newer things. I don't blame them for that in the slightest. But you don't need to crash a franchise into the ground just because you don't want to work on it any more.
If I were only allowed to give just one reason on why I hate the ending it would be because it destroys what made the ME universe such a great place. I could live with an ambiguous ending where it's left to the player to decide what happens next, like that fan edit where Shepard and Anderson stare out into space as the fade to black hits. I could even live with an ending where the entire attacking fleet is wiped out, and Earth destroyed but it's still a victory. This wouldn't be what I prefer, but I could still accept it because at the end of the day it would still feel like it was worth it.
It's analogous to not being able to use the warp drive in Star Trek. It robs the series of the wonder, exploration, and scale that was the backdrop to the story. It turns it into a smaller and uninteresting place, and kills the magic of the setting. Ultimately, any victory feels so hollow - even forgetting all the starvation, war and chaos that would surely follow if you analyse it for a second - that it's just not worth bothering with.