As for most efficient use of the resource, well, consulting my spectrum analyzer, ISM bands are winning by a mile and we should want more of them.
ISM is tragedy of the commons; make it free, let anyone do anything and it becomes junk. Carriers need something they have exclusive use of.
Carriers don't need 6GHz for backhaul. They have fiber and cable and (other) microwave. Not to mention the ability to shape their own links with antennas and beam forming and do a good job of it rather than a "default job." What they don't have -- and shouldn't be given under any circumstances -- is the excuse to build a moat in the bustling public park.
On top of that, mobile data is quite expensive in the US, so the only time I have data when out and about is... when I'm on free public wifi networks (which is most of the time). So I don't see much reason to give more of a monopoly to mobile providers. I honestly don't even see a use-case for cell service outside of super rural areas; the only reason I even have it is because it's necessary for MFA. Cell providers are legacy tech as far as cities are concerned IMO.
To me it'd make way more sense to me to let wifi have more bands with stricter limits on power levels, and any exclusivity should be to municipalities who can contract with companies to build and manage their infrastructure.
It's not 2015 - that narrative is long dead. There are countless options for unlimited mobile data (5G, with hotspot) for $15-$20/mo.
It's true that there's no single service one can sign up for and you have to bounce around cafe and Xfinity and whatever "Free WiFi!" networks are being offered. Which is definitely annoying and it's nice to have a single company sell you service in a neatly packaged "phone" product.
But again, trying to phrase that as a technical point is ridiculous. Free bands are just plain better, technically. You get more data to more people for less money using open spread spectrum protocols than you do with dedicated bands. Period.