There's a much, much greater chance of the A's to Vegas completely falling through than them moving on after a decade. If indeed a billion+ dollar, team-owned, baseball-only stadium is built on the site of the Tropicana, it's gonna be there for the long haul.
And as others have said, the number of realistic expansion/relocation spots is small. Probably a 3rd team in Texas (Austin or San Antonio) and either Nashville or Charlotte (maybe both if the Rays need to be moved). I don't think Portland will work, Vancouver
definitely wouldn't work; maybe Salt Lake City? But they're probably getting an NHL team soonish, and supporting 3 major franchises in a metro area that small is probably asking a lot.