LOL, DISH squatted nationwode spectrum for years and it wasn't until the tmo/sprint merger that they did more than build a single tower in Colorado. I don't think I've ever seen the FCC seriously enforce the buildout requirements since any license holder can say 'but its hard we need more time/money'
The issue you have to surmount is this reduces the value of the licenses in the short run. Which means less cash for the seller (the public) now versus a recurring productive asset.
The useless response is to decry hyperbolic discounting. A productive response would think through how to design the auction such that the public would prefer to have the productive, recurring stream of revenue versus some shiny thing today.
Well, that assumes the public isn't really benefiting from the products and services that can actually take advantage of that spectrum. Making less in license fees is probably a good trade-off if your phone is faster or you get interesting and affordable satellite services.
It hasn't ever really been an issue with any spectrum that they've bought in the past, and it has only ever been a concern with mmWave spectrum, because the costs of coverage are much higher than were ever anticipated with their tiny effective ranges. Anything below 3GHz seems to get built out and used extremely quickly.