Here's the article that I referenced upthread.
If the Athletics and Oakland put aside this pesky lawsuit about the new ballpark and the team stays in Oakland, after 2020 it will have been in Oakland longer than it ever was in Philadelphia, believe it or not.
As I said, Nationals fans are like Braves fans. They'll probably come out in force and draw over three million next year and may even show up in some degree of force when they play the Phillies at CBP, though not to the extent that Mets fans do (most of them are New Jerseyans who believe that going to NYC is a drag, so Philadelphia is their Mets homefield). And, because the Nats were a somewhat likable and compelling team this October. I think some casual fans may adopt them as their fulltime team, including more than a few here in the Delaware Valley who have always been "meh" or lukewarm about the Phillies, similar to what we saw with the Cubs in 1984, the Braves in 1991 and Red Sox in 2004 when a lot of people became fans for life of those teams.