[Arnt]

It's not really about cost. It has implications on cost, but…

Some spare parts have been out of production for decades. If one of those parts needs replacement, then keeping an exchange running may now need replacing much more than just the broken part, or the technicians may try using parts from other exchanges (the number of landlines is much lower than at peak, so there are spare spares in many exchanges, so to speak). That's an expensive way to maintain a network, obviously.

VoIP infrastructure is more fragile than copper assuming equal hardware age and comparable maintenance regimes, but I assume that the lack of spare parts changes that.