Especially being the Eagles being the Eagles, the Flyers seemingly on the way up and all of the buzz that has come over the Sixers over the summer. If the Phillies head into a rebuild, I could easily see a situation like we saw in the late-'90's-early '00's when Philadelphia didn't tune into the Phillies until after both the Sixers and Flyers finished and tuned out once Eagles camp started, times when the Phillies' season attendance struggled to top 1.8 million (and often finished south of that number).
Even after the Phillies got good again, it took the Thome signing for it to crank up again (and some of that may have been inflated by the Vet's nostalgia-induced farewell season and the first season at CBP). The last good memory that Phillies fans had prior to 2001 was 1993 which, as fun as it was, was also an unfulfilled dream. The fans came back quicker at the end of 2010's when the most recent good memory was the 2007-11 run which included 2008's title. The current future "last good memory" will also be chempionshipless, so that may make it slow for fans to come back.
The Phillies largely lost Generation X in the Delaware Valley in the '90's (aside from them being occasional bandwagon fans). The same will probably happen with Generation Z.
As much as people here on this forum look down upon Philadelphia fans in general, Philadelphia fans can spot a competent--and incompetent--front office from a mile away. The Eagles are arguably THE model well-run NFL franchise. The Flyers just might have finally threaded the needle of finding FO leadership that is both filled with Flyers guys AND competent forward-thinking executives (as opposed to guys who were stuck in the past or just not good in their jobs). The Sixers--after The Process was long bungled by Hinkie, Colangelo and Morey--seem to have found themselves an absolute crackerjack GM in Gansey. Meanwhile, the fans increasingly view Dombrowski is not terribly sharp or a guy who the game and business have passed by and is driving the franchise over a cliff while Middleton appears to be completely oblivious to that out of personal loyalty to a guy who has basically become his crony.