Speaking from a DOT perspective, most states use 3000-3500 psi design strength concrete for sidewalk (with the results often much higher). 4" thickness is sufficient for standard sidewalk, 6" in driveways. The material is plenty hard to resist shovels. As drizgirl said, the freeze-thaw cycle and road chemicals have a massive deleterious effect on the concrete, and WDW can avoid those almost entirely (grumble, grumble).
Most concrete has a design life of 50 years, though it can last much longer with proper maintenance. What might be interesting to discover from some insiders is the reason for the sidewalk replacements that have been occurring throughout the park. It certainly makes sense in the MK where you're seeing repairs at joints and whatnot, but I'm also seeing them in Epcot now. Most recently between Germany and Italy, if I recall. That crete is only 36 years old. Was it falling apart prematurely or was it for more aesthetic reasons, like fading color?
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