If they really want to set it in a new galaxy, having races alive that are extinct in the galaxy of the trilogy is only a matter of how they got there.
Like I wrote before, they could come up with some freak accident that occured at some point while the crucible fired or that some got away in the refusal ending.
I dont think it would be a law if they remove a race from one galaxy they have to remove them from all of them.
Though it seems at this point, guessing is all we get for a good long while and in the end, people will be upset.
Depending on what will happen in the next 2 years will determin the fate of the franchise. I dont think people would forgive a debacle like the endings of ME3 again.
Even though the EC supposedly eased things a bit, reading all over the web how many are still pissed... it could really burry the franchise for good.
I do have a bit of hope left though, since BW admitted to underestimating the fans reaction.
I still do believe that "Yeah... remember? War happened. Let's never speak of it again" would be the ultimate insult, but I can understand why some people prefer that.
To me the Mass Effect Universe was all about characters. It's what BW does best. And having the ones we loved/hated reduced to a mere side note is the worst I can come up with. I am one of those, that loathed the endings and picked with destroy the lesser evil. I am not ashamed to admit, that the Mass Effect Trilogy was THE best Series of games I have ever played. And that was because of the Characters (including my FemShep). And while I dont have to play as Shepard again, I want them sent off with something worthy and for that, they have to put in work. I am 100% certain they could not care less about the characters but it is what I care about and thus at least my 70 Bucks are not yet a certain part of their revenue. And yes, I know that those 70 Bucks wont matter in the slightest. I dont suffer from delusions. All I am trying to say by this is, that at least for me, there has to be more in a Mass Effect Game than just marginal conditions like Races, Gear, Design when it comes to continuity. I am not saying that it does not work in other franchises, just that it wont work for me in Mass Effect.
Edit: And to go a bit insane again, I would not even mind them saying "We are sorry, we wrote outselfs into a corner. We will release a revised EC with endings that will work better with the new game and this time we will listen to the fans. All of them. From the best to worst possible ending".