The mineral insulated wiring of the 1920's is likely still OK today, but the rubber is well past it's use by date, PVC has had problems, the plasticizer can leach out making it brittle, but this is rare, also if over loaded it can be damaged, but in the main PVC cable lasts a long time.
My granddads house wired 1920's had two 15 amp sockets one in hall and one on landing, iron plugged into the lights, dads house 7 sockets, one in each room and on landing, that was 1954, by 1978 when my first house built, still only 4 circuits, sockets, lights, immersion and cooker, but many more sockets, think there were 6 in the kitchen alone, by 1985 when this house built two lighting and two sockets supplies, lights split up/down, sockets side to side.
The introduction of 30 mA RCD's resulted in sockets also split up/down so that the lights and sockets for each room are not on the same RCD, idea is should you get a shock from socket supply you will not be plunged into darkness, but the distribution board has also changed, the so called high integrity so some circuits on RCBO's so sockets again split side to side, which means if there is a problem don't need extension leads up/down stairs, more even loading, and less wire used so better loop impedance.
There are other factors, like cables being run in the cavity wall, then insulation added, so each premises has to be assessed, my dad's house was rewired, but some items were left, the kitchen had been rewired before the full re-wire, and the wet room was also new, so those area were not done. But the rewire resulted in loss of supply to loft except for lights, and reduced supply to garage, and some walls with no sockets due to being papered and hard to redecorate after.
I did not re-wire it myself, main reason was speed, costing £600 a week accommodation for mother while being rewired, so speed was important. At one point there were 6 electricians in the house, I think the guy under estimated how long it would take, seem to remember £3000 and in North Wales and as said some bits not done, detached 3 bedroom house.
But pricing is hard, the different firms suggested different levels of re-wiring, some included LAN cables and TV, some included making good, the one I used did not include making good, however they were good in considering the work needed to make good after. So yes same as cost of second hand car, as so many variables. Council houses were re-wired all surface near where I live, and each house took two days, all done the same, and if tenant had changed things they were charged to correct them, for example if they had fitted a chandelier, it was removed, dumped, and tenants charged.
Today rules don't allow trunking where it could fall off in a fire, and so surface wired re-wires have gone up in price, plus metal consumer unit, and so often it can't be fitted in old location, dads house consumer unit under the stairs, I don't think it should have been fitted there, but was not my decision, it was put where old one was, which was silly as mother could not access it in wheel chair. This all reflects on the price.