Does Don hit rock bottom after Megan dies ....
There's one season of Mad Men after this. Unless the arc that Weiner plans is that Don finds redemption and lives happily ever after--and that seems extremely unlikely given the show's storyline to date--Megan has to go.
Divorce has been done multiple times on the show. So has suicide. And cancer. At this point, all signs point to a murder.
And, yes, this isn't CSI. However, as I mentioned before, NYC was an increasingly dangerous place back then. Furthermore, there has to be a reason that the scenes with Megan are always about violence lately, even if its just sirens in the background.
I like the character but from a narrative standpoint, I think she will--and has to--go.