Efficiency isn't as straightforward either. You're still being fed by 120V/230V AC, so you're going to need some kind of centralized rectifier and down converter. It'll need to be specced for peak use, but in practice it'll usually operate at a fraction of that load - which means it'll have a pretty poor efficiency. A per-device PSU can be designed exactly for the expected load, which means it'll operate at its peak efficiency.
We also don't use 5V DC grids because the wire losses would be horrible, so a domestic DC grid should probably operate at pretty close to regular AC voltage as well. In practice this means the most sensible option would be to have a centralized rectifier and a grid operating at whatever voltage it outputs - but what would be the point?
As to PoE: I personally really like the idea, but I don't believe it'll have a bright future. For its traditional use the main issue is that there doesn't seem to be a future for twisted-pair beyond 10Gbps. 25GBASE-T might exist as a standard on paper, but the hardware never took off due to complete disinterest from the datacenter market, and it is too limited to be of use in offices and homes. I fully expect that 25G will arrive in the home and office as some form of fiber-optic interconnect - with fiber+copper hybrid for things like access points.
On the other hand, for a lot of IoT applications PoE seems to be too complicated and too expensive. It makes sense for things like cameras, but individual lights, or things like smoke sensors are probably better served in office/industrial applications by either a regular AC supply or a local DC one, plus something like KNX, X10, CAN, or Modbus for comms: just being able to be wired as a bus rather than a star topology is already a massive advantage. And for domestic use the whole "has a wire" thing is of course a massive drawback - most consumers strongly prefer using Wifi over running a dedicated wire to every single little doodad.
