For shorter ranges because higher frequency.
For average per-bit cost, because more bits/sec in the higher frequency encoding.
But, for IoT there is the burden of turning the RF segment on, finding lock, sending. So, for small 'grams' of message it won't necessarily be as good as the per-bit cost for streaming data or long lived associations. And, shorter ranges: You will be deploying more base stations. Albiet with lower power budget too, but .. Nothing is free.
From what I understand, the range of a single 5G base station is significantly lower than 2/3/4G so they are having to install new cell sites in major towns and cities just for 5G coverage.
The focus on millimeter wave is large part due to existing capacity problems in high density areas, or for high bandwidth applications.
However I've seen that there is proprietary protocol symphonylink from LinkLabs, that uses LoRa. They claim that it allows for having every frame ACKed, even bulk data uplink (OTA firmware upgrades). If so, I would tend to pick such technology for pond water telemetry.
But wouldn’t it work for this use case he had?
I believe that in the long term and as the number of devices per unit of area increases, licensed technologies will be the only viable option.
A solar powered system out in the open should, theoretically solve the power issue.
I worked on a pilot project with a state government group that wanted remote monitoring of water levels in wetlands and ponds spread across a few hundred square miles of mostly-rural area. wi-fi obviously wasn't an option and getting a WAN mesh setup was a huge pain in the ass and had its own limitations. We ended up testing with 3G, switching on twice a day to report, but the power consumption and data costs were too prohibitive to be practical long-term. 5G power use per bit is supposedly going to be far lower than 3G/4G. Jury is still out on data cost.
I now work in the automotive industry where vehicle logistics can be an interesting challenge. Parking lots for completed vehicles awaiting transport outside factories can be spread across a square mile of land (more if remote overflow lots are used), with multiple locations. There are multiple use cases for inspecting, tagging, remote driving, etc. these vehicles using aerial or ground-based drones and teleoperation to significantly cut down on costs. However, those use cases depend on high bandwidth, low-latency connections and extending effective wi-fi to lots would be massively expensive. 5G is a pretty interesting option here.
As others have mentioned, vehicle-centric use cases abound, but there are some additional ones for industries like oil & gas as well.