It would be great if you could just call the people whiners for wanting a better ending, but you can’t. The ending they gave in ME3 didn’t even make sense for the universe, let alone closure, which is the culmination of a trend at BioWare that began a few years ago. That they can remove and add things to their games at the whim of the writers without regard for the already established lore. It was something David Gaider even said they had the right to do himself.
I didn’t personally play ME3, but I did spoil most of the game for myself. I promised I would never play a Mass effect game after 2. I felt betrayed from the very beginning and my concerns weren’t alleviated at all later on. I’d really like to go into detail, but I don’t want to spoil anything and I don’t have time to stick around. Suffices to say the more you care about the mass effect universe, the less and less you’ll like any of them after the first game.

I’ll leave you with these two articles that sort of reflect my feelings on ME3 and the franchise in general

Also yeah if they release DLC that fixes it, unless it’s free, they prove everything that’s wrong with the industry. There’s no redemption for them in my mind, time to move on and regain consumer trust with a new project.