Why do people are so insistent on the idea that a mermaid NEED magical/out of nowhere legs to be happy with a human?
There's always a more realistic solution:
Sarasa can use a wheelchair,the human girl maybe build a house next to the shore,Sarasa can live in a pool/bathtub in the human house,the mermaid being carried by the human to move on land,etc
It feels so cheap and unsatisfying when the mermaid just grow legs with no reason at all instead of solving this problem in creative/plausible ways