The impression I have of the PE is that it hasn't been developed in an ideal language and the 'clunkyness' shows at times.
Again I have to disagree. I acknowledge the 'clunkyness' where it does exist but do not think that's the fault of the language. The IDE front-end is written in Microsoft Visual Basic and entirely suitable for writing an IDE in. The original back-end compilers appear to be written in some version of 16-Bit C, the enhanced compilers in Microsoft Visual C++.
A problem with any code is that as it grows it becomes harder to maintain or alter, and the consequences of past decisions are not easily undone. I can well understand the argument of 'if it's not broken, don't change it' because that brings a lot of unexpected debugging, testing and use of resources which may be needed elsewhere.
So while any shortcomings have to be, inescapably, laid at the door of Rev-Ed, I can understand a reluctance to alter things when what exists is functional and does its job as expected, even if there are places where it could be even better.