Its extremely limited range and penetration is a benefit there because you can put up thousands of cells without them interfering with each other.
It was also meant for offices as a managed WiFi replacement. Stuff like that. In this vision every laptop would have 5G built in and could work anywhere. It just didn't take off because of the chicken egg problem. Not many phones with mmWave. Not many providers with it.
The speeds thing was just marketing BS as usual but mmWave did have benefits in niche scenarios.
It did not took off because the whole sales pitch was utter nonsense. Why would anyone put 3rd party 5G BTS inside their buildings and then pay for data, when they can use WiFi for free + they control the network so they can decide how data are shared between stations and which stations are allowed to be logged in.
I can see that 6G is pitching... exactly same thing. Which predictably is going to fail... on exactly same issue.
In india, jio is using 26ghz for wifi now instead. It's a service called airfiber. This don't even use any 5G. It's a weird custom hybrid solution where all the frames are wifi. Density here means that it's feasible to put the receiver on the rooftop of apartment buildings wherever you can get LoS. So in some sense it's been salvaged and those bands are actually being used. It's a fairly popular service.
URLL failed because it's main immediate usecase was supposed to be vehicle to vehicle comms. And auto didn't want telcos middlemanning there. But there is still hope here.