Well, I've covered this ad nauseum with durasteel and others, but I'm just generally opposed to setting any canon whatsoever on principle.
I think that refusing to ever revisit the entire galaxy of the Mass Effect trilogy sets a much more rigid canon than anything else on the table. You're basically saying that, as an in-universe truism, the whole Milky Way should be dead and gone, inaccessible to PCs, NPCs or their descendants from ME4 onward, forever.
If you want to mandate that players will never again be able to see or visit any place or character from the previous games, then your position makes "canon Refuse" seem moderate by comparison. Many fans expressed dismay over the extensive devastation of the galaxy in ME3, but you want to take it to an ultimate extreme and throw away whatever's left. Might as well give up and let the Reapers have it.
The Reapers were trying to destroy the galaxy. Not physically--there would still be chunks of dirt, ice, gas and fire swirling around a supermassive black hole like there were before, but the cultures, civilizations, and character of the place would be wiped away. Shepard was trying to save the galaxy in the same context. If, after the conclusion of his trilogy, the galaxy is so completely bollocksed that no more stories can be told in it, then Shepard is a complete failure as a character. That would be the worst indictment of the ending of ME3--that no matter what your character does, you inevitably lose the whole galaxy forever.