Under the Sea is one of the more underrated rides at Magic Kingdom. There's room for Beauty and the Beast without having to get rid of Under the Sea; it would just require some back-of-house infrastructure changes.
I look at Mermaid as a value proposition: the quality of the ride relative to the time required to wait for Mermaid is much, much higher than Seven Dwarfs, say, which is technically a better ride but commands ridiculous waits. I will never wait more than 30 minutes for Mine Train, whereas I'll almost always be able to walk onto Under the Sea and still get to ride a pretty charming, old-school dark ride.