I agree that it is a cult classic, but I also see the need for the updates. I would just be very surprised if the Mexican government would approve the 3 Cabs update. It really doesn't do anything to advance Mexican culture, it is not a popular Disney film, and it would be more costly than simply updating the attraction. Is it a possiblity (or even a good chance), of course! However, I think merely adding a new ending scene, some strong animatronics, new videos, etc. could be a much better approach. I also think that extending the attraction (which could be done behind the building if desired) could be helpful - small drop ala Maelstrom.
I just really would be disappointed by the character invasion to continue into Epcot, especially World Showcase. It almost says to me: our accomplishments as innovators or as cultures isn't interesting enough. We need Mickey for people to ride. That is borderline offensive IMHO.