From what little I've read of AA, I wouldn't want a new writer. Despite his disgusting behavior, he's clearly a talented writer and having anyone else conclude the series would probably mess it up. I know not everyone on this forum is willing to separate the art from the writer (which is fine) but Matsuki must be the one to conclude imo.
AA doesn't look like it'll survive in WSJ but not all hope is lost since it can very well end in another magazine (assuming Usazaki is fine with it). The same thing happened with Takeshi (it was apparently in it's final arc at time of the scandal). After like 3 years, it was later serialized in Super Jump (seinen mag.) to give a proper conclusion (a whopping 10 chapters) and hell even got the cover for it's serialization
comicvine.gamespot.com
I would hope that Usazaki will move on for now (i.e work on another series) and once some time has passed (at least a couple of years honestly) regarding Matsuki's scandal then they'll conclude in another magazine (ideally a lesser known one for a more lengthy serialization). If they were to hypothetically return as a duo and the demand was high enough then I don't see why they can't give it as close of a natural conclusion as possible. It's a shitty situation but don't give up hope.