How would the system know, reliably, that the IP address was manually set/static on the computer? Yes, you could evaluate the DHCP leases, but that only works if the host is known to have come online after the most recent reboot of the router. If the host is connected through an external switch, the port wouldn't bounce during a router reboot, so it would the host request a new DHCP lease. This could lead to a false-positive assumption.
Beyond that, the detection would likely happen rather early in the cycle... likely/hopefully before DHCP leases are issued, and almost certainly before the user attempts to login to the router for administration purposes.
I did specifically mention in my RFC:
IMO (I will admit: this is opinion -- I don't have data to back this up -- so it could be factually incorrect):
- the most common scenario where the automatic reassignment would occur is likely to be a novice user who is using DHCP on their computer
- Many novice users will be physically connecting directly to their OpenWrt router via ethernet (not through a switch) at least for initial setup/configuration.
- I view static IPs as an edge case scenario, unlikely to be common for novice users (for whom this feature would be useful).