Depends how high you build the bridges, but I imagine them being much too high to allow the riverboat to pass under. You could always dock the riverboat and turn it into Tiana’s across from Splash.That would require eliminating the Riverboat.
Personally, I would be okay with eliminating the Riverboat and Tom Sawyer Island because they are things I never do. I probably did them as a child (have no memory of it), but haven't been on the boat or over to the island in any of my recent trips. Fill in the river and that's a huge plot of land that could be used for multiple attractions, restaurants, etc.
With that said, though, just because I personally don't care about that area isn't a good reason to eliminate it (I'm also personally in favor of eliminating Jungle Cruise and using that whole area for multiple new attractions). Other people do enjoy it -- it's a bit like Carousel of Progress for me, which I absolutely adore and would be furious if it was removed but plenty of other people would be happy to see it bulldozed and replaced.
Totally agree with you. There’s no sense removing the entire island just because we don’t enjoy it when others clearly do. My point was that there are options available to both expand and retain some of TSI to appease both crowds. Nobody would be thrilled, which might make it a good compromise.