He'd be F***ing blown away and ALL of you know it.
I have seen the 1970s pics posted the past few weeks. The parks look sickly due to mack of vegetation upkeep. Eventually he may get around to maintinence, but imagine taking Walt down Sunset Blvd and on to Tower of Terror! Then for a ride on the safari. Next on Spaceship Earth. Finally on Splash Mountain (and no, I don't need smart@$$ comments about how things were broken a few weeks ago).
He would stop. He would cry. He would give you a hardy handshake. He would say well done.
Seriously, Disneyland did a full 180 in a matter of months before the 50th. It can be done in WDW too. Walt would look at the big picture though and be amazed.
As far as Epcot is concerned, I think Walt would adore it. It was never meant to be his Epcot. He would be a smart enough guy to understand that it is a homage.
I think he would be much more disappointed in the cynicism that we have towards WDW. That would.break his heart.
I don't mean this antagonistically, but you would be well advised not to tell people how they think, in regards to your first sentence. There's excellent reasons to think he'd be disappointed. Walt Disney was well known to have an impeccable eye for spotting details and flaws. Anyone who is aware of even snippets of what he was like knows all the accounts of his extreme attention to detail and drive for perfection and improvement in his endeavors. If average guests at the parks are seeing things wrong, it shouldn't need to be said that Walt Disney would be able to find far more wrong with the management than most regulars are seeing. The state of things has gotten so bad that even people who aren't obsessed with details can't help but see things wrong (and yes i will indeed quote "Ghetto Splash Mountain"). If you think vegetation is the sole problem with WDW, you're mistaken. Chipped paint everywhere, massive quantities of burned out lights, rust, fading paint, huge amounts of broken or only partially working ride assets. The list is massive.
It's clear from everything we know about Mr Disney that he would NEVER tolerate the sort of incompetent, amateurish, and absolutely unacceptable level of mismanagement that goes on at WDW nowadays. If he knew how everything was intended to function or look, he would be absolutely furious and disappointed with how the people running his company violated and destroyed his core ideal of quality of show. That's not even debatable. This is regardless of whether he liked or disliked certain attractions and parks.
Knowing what was once there, i also cannot imagine Mr Disney liking the current forms of SSE, Imagination, shuttering Wonders of Life, or even losing attractions such as World of Motion or Horizons to satisfy a need for cheap thrills by certain park goers. Though to be fair, i can't imagine he would have been satisfied with Epcot regardless, it was his biggest project ever to build a real futuristic city and the company quickly disregarded it. That would have saddened him immensely. Though the original 1982 Epcot at least contained many of his visionary ideals, unlike the current joke of a park (Future World at least). The one part of Epcot i think he'd like is World Showcase. Though he'd also probably have been "what the heck is this doing in Epcot?" I could see him being extremely impressed and happy with American Adventure. It really feels like something he would have made.
Animal Kingdom is the one park i'd speculate he'd at least appreciate somewhat, possibly. I think he'd definitely like the concept and that had he still been alive, we'd probably have seen something like it happen. Though again, he'd no doubt have been disappointed in what could have been but never made the cut. And of course the way management has defecated all over Everest and Dinosaur. Also can't see him liking Dinorama in any possible way.
And in terms of maintenance, you're equating what could be done with what will be done, which just isn't likely to happen. Disneyland was cleaned up, there's nothing to say that management for WDW is going to receive a sudden improvement. They aren't going to change anything when their profits continually soar. People are giving them no reason to change. Our resident insiders who know the inner workings of the company have also given countless indications that things aren't on any route to change.
So yeah he'd probably be sad that people were so cynical towards WDW. But for a different reason than you're stating. He'd share this feeling towards his own company, along with absolute horror and fury at the current leaders that betrayed his core ideals and work philosophy of quality of show. I don't have any doubt that he'd see eye to eye with the so called "cynics" in regards to maintenance at least. I hate to sound so sure of what a deceased person would act like and present it in such a militant manner, but we know enough about Disney to realize how quality of show and small details matter. What he might think about certain parks or attractions is another matter that we can't really know, but the way the parks are run is something he wouldn't approve of.