"Won't we all be on 5G (6G, 7G, whatever) by then? Stringing cables around will be history."
Do you think there is some sort of magical ubiquitous continuous 4G/5G signal in the sky, that somehow makes the rollout of a national fibre backhaul redundant to all areas of the UK?
As simple as I can...
The mobile 3G/4G signal strength shown on your device is just signal transmission protocol between the device and the nearest cell tower/mobile mast.
From there, the data is either sent via microwave then fibre backhaul or sent directly via a fibre backhaul or an older, slower data protocol if rural, in a non-fibre area.
An important point, crucially, Ofcom offers no guarantee/verification process that a mast stating 4G/5G has the necessary required bandwidth backhaul capability to actually transmit/receive data at speeds offered by 4G/5G for multiple concurrent users at once.
Ofcom should regulate this but don't, well not as far as I'm aware. (If you're an MP reading this, please force them to).
5G will work by offloading signal processing to the cloud (to make the local cell hardware cheaper) so uses a fibre backhaul for both signal processing/handover between cells and data.
Every 5G cell will require a fibre backhaul (via a commercial contract, but potentially within Openreach's local loop), hence why Openreach local loop fibre rollout is so damn important, in order to piggyback these fibre connections.
If there is no rollout of fibre, there won't be much (blanket) useful coverage of 5G, because the cost to provide the fibre backhaul for just 5G cells, rather than "piggybacking" fixed fibre broadband wouldn't make blanket mobile 5G coverage cost effective.
The way 5G generally (depending on the frequency) works is cheaper cell hardware (offloading the signal processing to the cloud), smaller cells and more densely populated, in order to offer an order of magnitude higher download speeds to more users concurrently.
Higher frequency 5G 3.4Ghz frequency range doesn't easily penetrate metalised glass or modern foil insulated buildings so can only really be used for street light style 5G cells densely populated, close by, with limited range, but potentially high throughput within a very localised area. (like Wifi).