It does seem strange to gut and completely change the gateway attraction (and the one everyone knows) at a park? Spaceship Earth is the second-most-famous WDW building and the only building at Epcot that's "household knowledge." However, I decided to try to look at this from a rational point of view.
Here's where I see the pros and cons of gutting the attraction:
Pros:
1) The ride system is a bit dated (essentially similar to the Haunted Mansion). Do you want such a dated ride system in
Future World's main attraction?
2) The story lacks "pizzazz." Note that I personally don't agree with this assessment, but many people might.
3) A slow moving animatronic-viewing ride with no real "thrills" doesn't cut it any more, again, for the most recognizable attraction at the park, and one of the first and last things people ride at Epcot, and the ride most likely to help mold overall opinions of the park.
4) Keep in mind that they can gut the inside, and build something new, and still call the building "Spaceship Earth." Much like how the Tree of Life contains "It's Tough to Be a Bug" inside. The actual "look" of the attraction would possibly not change at all.
5) Epcot is supposed to be continually changing.
Cons:
1) It is one of the original Epcot attractions. Many people consider it a classic. Would they take out "it's a small world?" No way. (However, Disney may think of it as a classic like Mr. Toad - a loved ride by a small subset, but most people want something more exciting.)
2) It's actually educational, which is getting more and more unusual at Epcot.
3) It just sounds wrong, like if they gutted Cinderella's castle and put a thrill ride in there.
I'm sure I missed some points on both sides, but weighing the positives against the negatives, I can see how Disney came up with the decision, both for economic reasons and to keep the park "fresh" and "modern."
Personally, I have mixed emotions - I do enjoy Spaceship Earth. however, I can see how something new and exciting in the most prime piece of real estate at the whole Disney resort would be a great kick in the pants for Epcot as a whole.
If I was appointed "person who makes all decisions for Disney," if I was doing what's in the best interest of the stockholders, I'd say build the new attraction. If I was doing what I thought was in the best interest of the park and myself, I'd have to think about it a lot more, but with the limited info out there, I think I'd lean towards gutting Spaceship Earth.