I completely agree with you that theming is what sets Disney parks apart from others, and one of the reasons I was underwhelmed by the Studios Park in France.
I also understand that LS/FL were built with the waterfront in mind, no debate from me.
I'm also a long time guest, going regularly for 40 years and appreciate the history (long live Horizons!). This was just my opinion that the river never mattered to me. The Epcot lagoon is used for night shows and doesn't impede guest flow.
It's felt underutilized to ME (again YMMV), the TSI was never appealing, the boat was never appealing, and I always found that the river constrained the walk way and it was a PITA to get from HM to BTMRR.
For those that enjoy the area, I'm sorry that you'll feel the loss. I'm just completely baffled by the response on here, because I honestly didn't realize the area had that many fans.