"...but fred bloggs setting up their own network using raspberries and off the shelf radio modules are going to end up operating on frequencies that are already licensed to other people"
True to some degree...but I would suggest that such "fred bloggs" 5G networks would be of very low power and hence unlikely to cause much disruption to higher-powered licensed operators.
[url]https://en.wikipedia.org/wiki/5G#Technology[/url]
Plus I assume that any "fred bloggs" networks would not be physically close to any high-power mobile towers so unlikely to be causing interference to the mobile tower.