Sea levels won’t rise all that fast.

Developed countries can rebuild their coastal cities every 50 years just a bit further from the shore every time, most already rebuild every 50 years or little more. Except for the ones that are already or almost underwater, but those are goners anyway.

Infrastructures like ports and associated transport, can also be rebuilt every fever than 100 years, they often aren’t even built with that long of a lifespan in mind, but can take much larger sea level increases.

As for developing countries… Bangladesh is toast, others will vary.

Overall, a 20m sea level increase is easily sustainable at a rate of 2m per 100 years or so.