The scoping issue I mentioned was function scoping, not variable scoping. The language would be better for organizing code if it supported functions as methods of particular classes of objects, or functions with private scope. As a codebase gets large, how well organized the code is makes a big difference in how easy and how risky it is to change things, so it really pays for your language to make code organization as natural as possible.
I am actually not able to find statistics matching up M databases with non-M databases at the moment, but I'd be interested in seeing something like that. I don't know what you mean about having to roll your own C extension to talk to your database.
I think that fast reads/writes actually are a reasonably differentiating feature--if you want something like that in JS, Python, .... you're going to have to drop down to a C extension, and you'll have to put a lot of effort into that being portable. In a MUMPS interpreter, it's already there and it already works reliably.