Don't really see the problem with adding Krug to replace Yandle. Krug is the better and younger player, so what's the problem? It would be like adding Yandle originally (except I like Krug better), and if Krug declines in 3 years, it's just the same problem we have today except we would have gotten 3 more productive years from Krug. Then you figure out how to trade him if he declines, and as you've pointed out, Yandle has trade value so Krug would also have trade value.
Obviously depends on what we may have to add in a yandle for krug swap, but I don't see the problem. That would be a home run. I would be shocked if it was yandle/krug straight up.