it's not private infrastructure
This logic ignores the fixed costs of supporting your "requirements"?
If some grandpa just wanted dialup, I'm not sure it makes sense to maintain that just because it "meets their requirements" on a cost basis when it was a volume product.
For all public/shared infrastructure it would be what meets most people's requirements.
And infrastructure has to be forward looking, the UK has already been squatting on copper for long enough.
You'd have a point if newer infrastructure did not meet your needs, not your wants.
There is a chicken-egg problem here - incentivising the move to FTTP makes sense, FTTC is not future proof and is legacy.
If 10 engineers on FTTC can be made to work on FTTP, then FTTC paying for the legacy attention it needs makes sense.