Of those middle-ground options, I can't think of any that don't involve revoking a lot of licenses and rendering a lot of expensive equipment useless. It's theoretically possible to manage spectrum dynamically, but you need a clear channel and smart devices.
Consider the digital television transition - it freed up a substantial amount of spectrum for other users, but it took many years and cost billions of dollars. Broadcast TV and radio is less significant today than it was in 2009, but there are thousands of spectrum user groups who would find it difficult to switch to dynamically managed spectrum, not to mention the potential safety issues.
The RF spectrum is a busy old place. We can carve out little bits here and there, but there just aren't many useful blocks left that aren't already being used by something quite important. Most of those allocations involve co-ordination with the ITU and couldn't be significantly altered without international agreement.
https://www.ntia.doc.gov/files/ntia/publications/january_201...