Which is why we probably leave for the Andromeda galaxy during the events of ME3 and not after.
It does depend on exactly when we're leaving, that's true, but I have a hard time imagining it's prior to the Tuchanka arc; at that point factions were either focused on the idea of actually beating the Reapers or were still in the running around with their heads cut off phase.
Right now the best guess for when we're leaving is after the fall of Thessia where everyone is both organized enough to put something like the Ark Contingency into effect and the situation looks desperate enough that we're going to need it.
If it's post Tuchanka, then we need to establish whether or not the genophage was cured at the very least. If it's post Thessia, then we need to establish how the Rannoch arc was settled; whether the quarians or the geth or both were saved.
If it's essentially within the first couple hours of ME3, then yeah, we don't need to establish any of that, but really, would just establishing the big decisions like those be such a bad thing? We aren't talking dozens of questions here; we're talking four or five big decisions that impact whether or not a species still exists in any meaningful form or if a species still has a sterility plague.