Don't worry. Even when it's sold it's never really sold. If someone with more money comes along they get your spectrum. Just look at the incumbent satellite uplink operators who had half of C band taken away from them and resold (to 5G telcos).

As for spectrum usage, I can only talk about my region, but I have been monitoring all of 70-1000 MHz at ~10 second intervals and storing the data since 2014. For periods of that I monitor 700-2700 MHz as well. The cell operators are not letting their bands sit idle as far as I can tell. It's nothing like what the top level comment thread implies re: ISM vs. Cell bands. If anything it's the reverse.

But that doesn't mean they should get this 6-7 GHz of new ISM spectrum the FCC returned the original owners: the people. They really, really shouldn't. And they shouldn't be allowed to operated unlicensed LTE in the ISM bands either.