I don't believe Arias is blocked by Mayer as Mayer could 3B/SS and Arias could play SS/2B. Arias won't be ready until late 27 most likely, so there isn't much overlap with Story.
If they deal Mayer this offseason, it's one more hole Breslow has to fill and it would probably be with a contract much larger than Mayer's. I think the FO would rather keep Mayer, but who knows.