RPL is a beautiful design in and of itself (its tendency to lead to write-only stackrobatics notwithstanding), but it doesn't mix well with the HP-42S design, in my opinion. I was never a big fan of the unlimited-stack patch in Byron Foster's Free42 adaptation. The patch was well done, but the result was just not a 42S any more, again, in my opinion. And of course a big stack breaks many HP-42S programs, and I and many Free42 users value that compatibility.
Which isn't to say that Free42 couldn't be enhanced in other useful ways, like directories, local variables, unlimited RTN stack, etc. But those could all be done without breaking the existing design or HP-42S compatibility.