Replacing the wires cost half as much as adding new wires. So I think it actually makes more sense to add new wires. It will give some level of redundancy for only double cost and you don't need to impact current electricity delivery.
Weirdly, how utilities were financed blocks retrofits.
Public utilities make money by building new stuff. Increasing efficiency actually loses them revenue. Eventually, we have to reboot their businesses models. Meanwhile, we need legislation (and subsidies) to compel retrofits.
Maybe with fiber transceivers, sure. But twisted pair is fine... Some of the 10G equipment doesn't go all the way down to 10M anymore, but just put your old 10M equipment on the 10/100/1G ports, and the 10G equipment on 10G ports. It can be on the same subnet, no big deal, as long as you're not running jumbo packets or at least only using jumbo with MTU discovery. (You can usually even run 10G on cat5e, as long as it's not too long, and not in dense conduit)