Perhaps there's room for improvement in the last mile. Copper is expensive, insulation is cheap. Wonder how much higher the tensions in electric outlets could be made while still keeping overall safety of the system.

For transmission lines, AAAC, AAC, and AACSR exist.

For residential grid hookup, aluminum has been an option for a long time.

While aluminum residential wiring has historical baggage of risky and bad practices, the problem with it is inherent fatigue failure. Copper should be required for residential structural high voltage wiring.

Locating, mining, and recycling more copper is necessary because demand is far exceeding supply. Perhaps at some point in the future, we may need to mine disused 20th-century waste dumps for metals and rare earths we just threw away and failed to recycle.

If a higher voltage were implemented, most/all of the switchgear and power lines would be retrofitted per engineering specifications.

Design values change, but not as much as you think. 1 cm for every 10 kV is reasonable between parallel plates. The dielectric strength of air is roughly 3 kV/mm.