Indeed.
Not necessarily, it varies enormously IME.
While true, it does not mean that a circuit as installed can't be carrying out the same job equally well now. You need to look at the actual situation rather than make bold statements from the comfort of your armchair.
I rewired a friends place that still had all its original 1956 rubber cable (plus a few add on bodges he had done over the years). It was surprising that the state of the cable was remarkably sound. No signs of flaking at the termination points, and it passed an insulation resistance test with ease. In many respects it was better than some of the cruft that had been added on with far more modern materials.
It was woefully inadequate in terms of number of socket outlets (5 in total in a 3 bed semi), but to an extent that was a saving grace since it made it highly implausible that the circuit would be loaded to capacity.