The big loss here isn't losing historical attractions or even the view.
The big loss here is that the river was the place to take a rest from Disney. This is where to go to when everything becomes overwhelming and you just need to get away from the shoulder to shoulder crowds, the screaming kids, the hyperactive oversensory of the rides and worlds, rude guests, the upcharges, everything for a half hour. If you needed to recharge or destress you could get on that boat and let your mind go before heading back to the rest of MK renewed. It felt like a immersive different world in itself and it's an intentional design, back when they were making this park they could have put the swampland water out further and built attractions where the river is, but they knew people really needed that so that guests can be calm and spend more without having to leave to get away. It might be hard for the Disney Adult who wants to live in an immersive Disney community to grasp, but the average guest can only take so much before being overwhelmed, and don't want to be immersed or swarmed with people the entire time. That's part of the reason Starcruiser failed, they overestimate how much the average guest wants to stay immersed without break.
That's an outcome that I think can't be expressed conventionally in spreadsheets. That's what these execs and the "everything needs to be 1 hour waits or it's not worth keeping" crowd here don't get. It was meant to be less crowded by design because it served its purpose of having guests escape the crowds when they need it so they stay in the parks happy to get more value out of them later. I know a lot of people who used it for that purpose, and it help both the guest and company needs. Now you're removing it for loud car vehicles from a massively popular IP which will bring the overcrowding, noise pollution, screaming kids, and general madness of the rest of the park here. There's now just about nowhere for people to get away from it at MK, especially for regular guests.
The impact of removing the river scenery and riverboat will show itself over time, just like not having any shade. They only look at a spreadsheet and shareholder opinions and see it doesn't have 1 hour waits and stampede-prone crowds around, so it needs to go. What they won't be able to find out immediately is when they find out guests are more stressed, overwhelmed, and wanting to leave sooner while spending less in later parts of the day. They aren't going to immediately get it because they'll see the stat of it being more crowded so it has to be better for the park. But they'll eventually see when guest response and individual spending at this park specifically decline. They might figure it out if they visit after this happens. But by the time they do it'll be too late to turn back.