but what has happened in ME 3 is going to be dealt with sooner or later
That's the point, though - it doesn't have to be. Chronologically ME3 could be the end of that galaxy's timeline, period. Forever. Whatever happened next was already explained by the Extended Cut epilogue videos - rebuilding, ShepReaper protection, or a green techno-utopia.
It was intended to be the end, after all. Mac said ME3 had big decisions precisely because they wouldn't have to import the world into a new story. Casey said right before ME3 launched that he couldn't see any future games being set afterwards. The Stargazer scene was set 10,000 years later, and Extended Cut showed us the consequences of the ending choice in intervals of months, years and centuries.
A new galaxy means they very literally do not have to deal with the consequences of ME3. There's no "sooner or later" if we never see the Milky Way ever again. That story would be done.