I understand that people don't like change, especially at Disney World. I think once people get past the grieving stage that they'll realize that there's still time to experience the attraction before it changes and maybe it will give them an excuse to go check out Tokyo
Disneyland. I also think more people will be excited about it once they see whatever they come up with and the kids growing up with the new ride won't care one way or another. They'll just see a fun ride with a Disney Princess.
Disney World is not an museum, it's a theme park and rides change over time. Even popular attractions have to change. If rides don't change and everything stays the same, I think it would get pretty boring.
Personally, I do believe this was primarily a financial decision. Disney is in the business of making money and the biggest problem is that the current property just isn't very marketable. Personally, I don't remember the last time I saw someone in the gift shop actually buy something. While we don't know the ride's maintenance schedule, I think it's obvious that it was due for a major refurbishment, which would mean that Disney was going to have to make a major capital investment in the tune of millions of dollars no matter what. The ride would no more money after such a major investment if they just kept it the same, which is a problem if you're looking to recoup that investment.
I think they looked at the success of Frozen Ever After and said, hey, that ride is super profitable now after we added Princesses. Let's do that to this other log flume that also needs refurbishment. Maybe we can do a desert party and sell a bunch of Princess stuff.