At my company, we write new MUMPS code all the time. Some of this is sanely written ANSI standard MUMPS, some uses the Caché extensions.

There are techniques and strategies for containing legacy code which are pretty well known in the community. At minimum, this would involve wrapping it in a sane interface. You'd want to isolate global variable state too. Alternatively in Caché you can wrap it in classes (whose semantics are close to Java/.NET)

It's on our list to rewrite the legacy code "some day."