Well, does The Princess and the Frog really qualify as a "franchise"? Doesn't a "franchise" have to consist of more than one movie, or at least one movie and a TV show?
Depends on what the bar is for a "franchise". If you're talking the literal definition, generally more than one movie is indicative, but if there's merchansiding potential worth capitalizing on (even if not quite enough to greenlight a sequel) then the bar is met.
The crux of this whole thing is Disney's interest in doing away with Song of the South. That's the definitive starting point. Disney does not wish to perpetuate its usage in any form. We've reached the point where they no longer wish to exempt Splash Mountain from that.
From there they went "if we're gonna get rid of the Song of the South elements in Splash Mountain, then what could we replace them with?" - So they turned to properties that 1) Make money, either through movies or TV or merchandise, and 2) Could be made to fit on the bones of the existsing Splash Mountains. In that order.
Princess and the Frog has merchandising behind it - more than Song of the South ever did or Splash Mountain ever will. Is it their biggest money maker? No, but it does make money, there are cultural incentives to add it (internally
and externally, lest it be thought I'm saying the public made this decision for them) AND there isn't a more obvious property that covers all the necessary bases. Not one that follows their order of operations, at least. You could find another property that makes more sense in Frontierland, but it won't be one that makes as much money as Tiana, for example, so the scale doesn't tip.
The setting of PATF also has potential to work nestled up to New Orleans Square at Disneyland (in theory, at least), which is also a consideration - they don't want to redesign the different Splash Mountains into two different new attractions. So what's good for the Gander is good for the Goose, in this case - they're splitting the difference on what could pass on both coasts.
And THEN, probably after ALL that comes the consideration of what could actually make for, you know, an actual good attraction. Princess and the Frog has some fun characters, good music, and interesting locations that have the potential to translate into a fun theme park experience. AND a good portion of the movie takes place on waterways, and therefore could be served by a flume ride. Bingo. Or at least, you know, close enough.
Considering how many boxes there are to check, and the order in which Disney prioritizes them, there really isn't an existing property better suited to replacing Splash Mountain - which, again, is the pin on which this whole thing turns and that really bears repeating. Their interest is in doing away with Splash Mountain in its current form - and
because that's decided the question then becomes what to do with what will be left of the ride. Which is SUPER different from the flow of logic in the opposite direction, which is what many people here seem to think.
Kicking the Brer's out of Splash was the
first domino in this chain, not the last. Therefore, PATF being brought in is a solution to a foregone conclusion, not an assertion over the ride as we know and love it. You know?