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So really there's the lie in the current availability figures:
the system doesn't provide the advertised capacity, even allowing asymmetric delivery. There's non advertised contention.
The 80% or whatever availability only works if circa 30% of households use the service. If the take-up figure was say 75% of households sign up the fast broadband equipment would need major overhaul in the exchanges and at cabinets.
In reality there's only one third of 80% delivery of service and that is subject to contention.
It is only a lie if you don't understand the difference between coverage and capacity.
Telecoms - as a century-old business - has always been about providing capacity to meet statistical needs, and expanding the capacity as the statistics show you are getting near the limit.
In the days of Strowger exchanges, you had a limited number of selectors for listening to dialling pulses. The dial-tone wasn't just a nice tone to listen to ... it was a confirmation that a selector had been allocated and connected, so you could start dialling. At busy times, it could take a few seconds.
Likewise the number of junction and trunk circuits - to connect you to other areas of the town, or other parts of the country - aren't infinite, and could run out. There is a slightly-asymmetric version of the "busy tone" that was used to indicate that the network was out of equipment, rather than the far end being busy on the call.
If an operator over-deploys equipment, it just sits idle, unused. A total waste of money that could be better spent elsewhere.
Back to our modern day broadband. Why would we expect BT to deploy an FTTC cabinet to be immediately capable of serving 100% of properties? We know that 20% don't want broadband of any sort, and 20% are happy with Virgin, which leaves only 60% that you need to target. And so far, it looks like two-thirds of that target group have been happy to stay on the cheaper ADSL-based services.
On top of that, BT know their equipment has a lifecycle where it works its way gradually into service (starting with appeal to early adopters) but equally has a gradual decline from service too - those early adopters will have given up FTTC ports to jump onto G.Fast ones instead.
That is considering "capacity" as the number of ports available. Considering the bandwidth available brings in a whole new set of things to watch. Sky, for example, brought in a whole new generation of Cisco routers earlier this year, to cope with the expansion of traffic on their core network (none, incidentally, at the behest of Ofcom).
Responding to demand correctly - by careful analysis of the statistics - is the difference between making a profit, and going flat bust. Analyse, and respond appropriately ... where needed. That is true of both the access, backhaul and core networks.
