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That much infrastructure would be problematic in localised areas no
Part of the problem is that millimeter band radio signals barely penetrate buildings, leaves, trees etc. Struggles with fog and rain and basically dies if it snows. Difficult to transmit omnidirectionally too.
I suspect that the plan isn't to roll out the full fat service in large areas, perhaps just in stadiums etc as Soul suggested.
@chak you're also probably experiencing sub-6Ghz 5G at the moment which is not full fat 5G. Three own the mmWave 5G spectrum in the UK but it's super hard to implement and requires antennas almost literally every 200-300m to work properly.
When that's rolled out at large urban concentrations like stadiums, rail / airport or shopping hubs, you'll see 1Gps connections which will be insane.