There is a bit more to the setting they will be losing other than location though. You have admitted yourself it is unlikely to have Geth and possibly not Quarians either. Would Batarians be invited? There would be question marks over Vorcha and Krogan (had we not seen one). Tech dependent species like Volus would struggle as the arkships would have to specially cater to them.
Not only that but because of the upheaval the relationship between all the species would be altered as well. There would be no potential Krogan threat. The Batarians wouldn't be martyrs. The Volus won't have any capital. Quarians would not be seen as gypsies. All the species would depend on each other so ancient grudges would have a lot less pull.
All (or at least most) the corporations that manufactured technology and weapons would be gone. The news networks we know would be gone. STG would be gone. The Alliance would be gone. The Ardat Yakshi would be gone. C-Sec would be gone. Laws and treaties would be gone.
Individual characters would be gone. Krogan and Asari would be gone. Thousands of other little individual things would be gone.
Minor creatures like Pyjacks, Thresher maws, Yahg etc would be gone.
As an aside we have seen N7 designated soldiers. We have seen Krogan. Why would these two groups deliberately run away from a fight leaving others to die?
Vorcha, maybe maybe not. Batarians also maybe, maybe not. Really, it boils down to what the writers decide. It would't be hard to rationalize Batarian involvement or vorcha stowaways.
How the relationships between the species will evolve is really part of the appeal to me actually. I mean, ME3 was pretty much all about overcoming those differences, so I'd think the old relationships wouldn't persist as they had in the Milky Way either, no matter the ending. Krogans are now seen as the saviors of Palaven. Quarians have their own home world and need not be vagrants. The Volus, along with most of the galaxy, virtually emptied their coffers funding the war effort. Every ending (Save refuse, obviously) talks about the galaxy entering a new age of cooperation. As I see it, this sort of thing is going to be evolving anyway. The setting is going to change in any sequel set after ME3, so we're not exactly losing that by going to a different galaxy.
Given how much they've been going on about how separate ME:N is going to be from Shepard's saga, I wouldn't count on returning characters (Indeed, they may want to avoid them. Small world syndrome is something Mass Effect has enough of).
As for other creatures (Pyjak, Yahg, etc.) those are things the writers may discard with... or take them along for the ride in some sort of ecological conservation section, but would the loss of such creatures really make this not Mass Effect for you? Some of them didn't even exist until latter entries.
As for the Krogan coming on the Ark ships, it might not have been their choice. One of the ideas I brought up for populating the Arks was for the cabal to lure in refugees and others with a Sanctuary style scheme. Or maybe they were mercenaries, hired as bodyguards and possibly not told the full extent of the plan. There are plenty of ways to get them there. As for N7, we still don't know what N7 means in ME:N. It could have evolved to hold a new meaning, like those unofficial "N7" operatives of the ME3 multiplayer. So really, we don't know.
I guess what I'm trying to say is that I've accepted that ME:N, whether set in a different galaxy or not, is going to be radically different from its predecessors in terms of setting. I don't think setting it in this galaxy adds anything except the unnecessary burden of dealing with the endings and the possibility of minor character cameos from previous games (Which, frankly, I have no particular desire for). If it isn't radically different, and we're given the same rehashed conflicts Shepard supposedly resolved in ME3, that would be very disapointing.