That's more of a physics thing.

For building penetration you need low frequencies but those low frequencies have less bandwidth available because the bands are smaller. At the same time bandwidth needs rapidly go up, making those low bands fuller. Because there's a limit to how many bits per second you can get out of a Hz of spectrum.

What would really help is the easy provisioning of signal boosters. A bit like the sure signal thingies that Vodafone used to sell but multi provider ideally. This could be incorporated into the standard. Then establishments could easily include a booster just like many of them offer free WiFi now.