I get the argument, but the networks seem to be operating quite inefficiently anyway. They have been given a tonne of spectrum over the years and yet they don't tend to address areas of concern over coverage. They could put in more small cells to improve coverage, but they just want to maximise the utilisation their wide area coverage frequencies. I live in an area where you'd expect fantastic coverage, close to a globally significant city and yet not too dense population. A 5G speed test will often get you DSL speeds. Leaving a few channels, or even allocating guard bands to low power 3G seems like it won't significantly impact the networks and at worse inspire them to invest better in their networks. Scarcity drives innovation afterall.