Lease the frequency
Why can't the government just lease the frequency for say 5 year periods with the current holder guaranteed the lease again so long as they meet minimum requirements?
Place minimum requirements on operators for usage of the spectrum, and coverage, with the requirements becoming more stringent over time. This enables operators to build up nation wide coverage over time, ensures people don't hold on to spectrum they aren't using and reduces the one off cost to operators. Longer term, they may pay the government as much, or more for the spectrum.
