Safest at the Magic Kingdom, at least in my opinion, is probably the Haunted Mansion. It has had some small and some not so small changes and alterations since they completed it, and there will no doubt be many more as the years roll on, but I sincerely doubt we will ever see the back of it. Pirates is in much the same boat.
As for Snow White, they aren't closing an opening day attraction per-se. That would be like closing "Under New Management" and claiming you are closing an opening day attraction, which was the wholly different "Tropical Serenade". Was Snow White ever in the same popularity league as Horizons (which I never saw) or the Haunted Mansion?
Everest will certainly be safe at DAK because, for the most part, it is the only reason that a chunk of its guests actually visit the place. That and the fact it is brand new...
Interesting question, though. I've often thought this myself. To be honest, I always wonder whether Walt Disney World will still be standing in 100 years time and, if so, what it would look like. The infrastructure was certainly not built to last as long as that! I know that some of the show-buildings at the Magic Kingdom were built with a projected life-span of some 50-60 years.