Neither Walt's original concept nor the initial realization of Epcot were sustainable as a theme park. Both relied on a type of cross-corporate partnership that has becoming much more rare today. Plus, the whole idea of "better living through (corporate R&D developed) technology" probably has far less appeal to the average Disney World guest of the present. In this day and political environment, can you imagine if Disney were to re-release something like "Monsanto's House of the Future" (Yes, I know this was an attraction of Tomorrowland at Disneyland, but it was built in the same spirit of Epcot)? Greenpeace would probably accuse Disney of genocide before the press release was even finished. For a non-Disney example of how out of fashion these sort of things have become today, when was the last time you got excited about an upcoming World's Fair? I had to look it up in Wikipedia to realize that these events even still exist (although they call them "Expos" now).
So, although I miss the spirit of old Epcot, both in its hope for the future and its exploration of the wider world, the people have clearly voted with their feet. They want to be entertained in a familiar way. I would much rather have a loosely-themed but well-attended park than a crumbling relic of another time that gets mostly ignored (and by extension, causes that much more crowding at the other parks).
With a few exceptions, I have rarely been disappointed by the various changes Disney has made over the years, so I suspect I will enjoy the new Epcot just as much as the old.