WISP is not symmetrical. Wireless isn't symmetrical. Nor is cable/dsl.
DSL splits the available frequencies into downstream and upstream, such that usually much more frequencies are allocated downstream. Wifi on the other hand does no such thing. The clients and the base are exactly the same and will send at the same bitrate. With wifi you can send or you can receive, but not both at the same time. Which means wireless is perfectly symmetrical in that you can either download at full speed, or upload at full speed, but not both at the same time.
WiFi 6E should have MU-MIMO which is something the WISPs have had for a few years, but not on equipment that speaks 802.11 WiFi. That protocol wasn't really designed to do 1-15 miles, it was designed for 1-150 feet. That doesn't really have anything to do with upload, I don't know where you got that.
It is true that wifi is designed for short distances. That has not stopped WISPs from using it for longer distances anyway.
Wifi 6E (802.11ax) has centrally controlled OFDMA which is used to assign resource units to clients. This is completely different from previous wifi versions. It means the selected frequency is split into 26 to 996 smaller frequency bands, which can then be allocated to clients as needed. This allows clients to send without any risk of collision with other clients and clients can dynamically ask the base for more resource units, if it needs to send much data etc. All of this is more like 5G than previous wifi. For a WISP it should result in drastic improvements to upload, since you will have less collisions and multiple clients sending at the same time.
>As soon a certain threshold is reached, higher speed will not cause more utilisation of the airwaves.
That's simply not going to happen. Do you think the cell companies stopped deploying towers, too?
I am not sure what you want to say with the comment about cell companies. I am saying that providing 10, 100 or 1000 Mbps upload to my customers on our FTTH network makes little to no difference in the amount of upload that happens. It will be the same on a WISP - why would it not be? So when you go from dog slow upload to super fast upload, all that means is that the airwaves will be idle more percent of the time. And when you do have the occasional customer that uploads a lot, he will not step as much on the other customers due to OFDMA.
Since Wifi shares the same frequencies for up and down, having one fast will free time slots for the other.
Regards,
Baldur