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I would imagine it would be a very poor user experience. Especially on mobile, where you jump around 4G/5G/WiFi networks and get a new IP address each time.

or if the user allows JS in their browser, then just do some GPU/CSS/Font Collections,etc. finger printing and get the unique id, since all the major bowser vendors are not fixing this problem (except may be for webkit).

IP can change multiple times when switching between mobile stations and wifi hotspots. Plus IP may change for each new TCP request if your operator uses CGNAT

you can have multiple IPs at the same time.

The "IP changes" is a common fallacy - even the mobile IP depend on the datacenter(s) process it. There is no grantee there will be a designated aliasing, and effectively there could be multiple IPs for the same client interleaving.

Best things would be webauthn were you have crypto hardware like your smartphone

The IP can change, is not guaranteed unique (many people can be behind 1 IP). Imagine unstable mobile connections going over 4/5g or random wifi as you move.