That solves the Reaper problem but doesn't solve the ending problem. The writers don't want to be faced with the possibility that Andromeda is synthesized. You also don't want God-Shep to pop over to Andromeda when the fleet drops out of a worm hole or the arkship arrives and say "come on back dawg, there ain't no problem any longer. Don't fuss about the Khet, I'll sic Harbinger on them". From what we can tell it is supposed to be a fresh start.
I would prefer to say that the Reapers were incapable of intergalactic travel. Put a lore limit on their traversal mechanism somehow. You need to keep things clean and manageable from a lore point of view.
A lore limit would mean that an ark could only get to Andromeda by a mass relay (which, to our knowledge, doesn't exist) or a wormhole (which also feels a little contrived, if you ask me). There's actually nothing in the lore right now that would stop them crossing a distance of that size; between ME2 and ME3 they successfully went from the edge of the Milky Way to Earth.
I think a more plausible explanation, if the writers wanted to use one, was that the Reapers and the Catalyst were only interested in the Milky Way and confined their harvest only to our galaxy. Chalk it up to the Leviathans' original programming, or the Catalyst's faulty wiring, or the (maybe, wise) belief that staying in a galaxy whose evolution they can control completely is better than potentially finding new enemies in different galaxies.
After all - despite the discussion above, there's absolutely no evidence during the series that the Reapers travelled outside our galaxy. Everything we've been told is that they hibernate in dark space. Introducing the idea that they harvest multiple galaxies (or, worse, that the Crucible affected multiple galaxies) would only make the Reapers a huge plot point again, when I feel like a big reason for going to Andromeda is to get away from that plot point entirely.
(In fact, if I had to bet, the Reapers will barely be mentioned in NME at all.)