Of course ancient telephone wiring can carry 1 Gbps. The real question you always, always, always, need to be asking yourself is:

Over what distance?

Make that distance short enough, as has happened with FTTN, or FTTC deployments in a whole heap of places, you're basically building a network that's, and I'll keep this very brief, subpar.

Since you mentioned a UK context there, Openreach rolled out an upgrade that kept the last mile of copper but now just about a decade later they're rolling out Full fibre. Whatever argument copper had, it went out the window near enough a decade ago.