Yes, but to continue my Finding Nemo Submarine Voyage comparison, that only has a capacity of about 900 guests per hour. And yes you can simply "lose popularity." It is nowhere near as popular in the years after as it was when it first opened.
But, that's also at Disneyland, a park that has millions more visitors than Epcot per year but also quite a lot more attractions to siphon guests to. There's only a handful of rides people still want to do at Epcot anyway, which is where the real problem will lie. Hence the animatronics added to Gran Fiesta Tour, in a last ditch effort to hope people one more attraction is added to that list.