A grid supply status, could empower next gen energy saving home appliances. This could be implemented as simple as pricing, the ultimate incentive. Price would fluctuate on the wire, just like it does for energy market.
Why? I lived through one such change in last 30 years (switching from 2-pole europlugs to CEE 7/7). They are backward-compatible, but so would be the usb+power plugs.
We also switched from 60 Hz 240V to 50Hz 230V some time in 00s (don't remember exacty, it was a non-event).
As for incentives new electronic devices should simply be reqired to have these plugs and microcontrollers, like now we require them to have fuses and certain wire gauge, etc.
Why depend on a hack when you can do it right?
And USB isn't meant to operate over those long distances. So you'd either need some active component in each socket or devise a new standard.
Rewiring might be a problem (but it was the same with adding a ground wire).
If we have to we can do the data-over-power between the sockets, and use data lines from sockets to devices.
This would ensure devices don't have to adapt to that hack and eventually we'll have a clean solution.