But leaving the galaxy does make everything Shepard did pointless because it renders whats happening there irrelavent. There is no point in saving the galaxy in the first place if your just going to leave it forever and never return. Its fate would be ambiguous (the bad kind of ambiguous) and the EC only gives a breif glimpse into the future its not the end all be all state of the galaxy. Its also worth noting that Bioware did make certain things canon in the past e.g. Udina being counciler, I wouldn't be suprised if the do it again.
I didn't understand this point of view when you first voiced it months ago, and I still dont.
Presumably, this "ark" (if legit leak), was a contingency plan if the Crucible failed. The races of the galaxy likely wouldn't know about it at large. Otherwise there would be chaos, everyone would be wanting a ride out. So saving the galaxy mattered TO THE GALAXY. Even if they knew about it, saving the galaxy still mattered, because the Reapers needed to be stopped. Even if the ark was a post-war exploratory measure, saving the galaxy still mattered...because the reapers needed to be stopped.
Again, you aren't upset by the fact that your choices dont "matter". If you're really honest about it, you are upset because you wont see HOW your choices mattered.
You may find that to be a subtle distinction, but it certainly isn't. You wanted a true sequel, not a sidequel. In both, your choices matter. But only in the sequel do you see how they mattered.
Other people, like me for example, dont care. I am perfectly content with the self contained story of the trilogy and knowing my choices mattered for the characters of the trilogy. I suppose its for the same reason that I am perfectly content with giving meaning to my life even though an afterlife probably doesn't exist. Same rationality. Both the journey and the actions you make matter, even if you dont see the outcome of them.