FWIW, many years ago, I had to switch from RMAIL to mh due to some of the factors Gregory mentioned, especially infrequent but troublesome corrupted rmail files and problems with multiple concurrent clients. These problems are rare when users only use a single device to read from local storage, but even 25 years ago I was using multiple geographically separated computers and shared networked storage. RMAIL also suffered, performance-wise, as email volume increased. Certainly, some of these problems were more due to movemail than BABYL.
That said, I think the existence of very old email clients such as nmh (and mh-e) and alpine/mutt and newer clients such as notmuch and mu/m4e provide enough alternatives to rmail that (I suppose) the interest in an updated rmail is very small.
I hope that helps,