Sorry, I can't hop on board. My opinion is that implementing anything complex and new in ANSI MUMPS would be irresponsible given the better options available.
> It has limited commands and is whitespace-aware
Limited commands to a certain extent just means a more dangerous language. Being able to give functions private scope is something that's really important to me personally, as is having named constants, and objects with methods. About whitespace awareness, I consider it a fascinating design choice that the difference between one or two spaces between tokens within the same line of code can mean different things.
> Newer developers can jump right into M, precisely because it lacks higher-level abstractions like first-class functions or closures.
The language does not support first-class functions, but it does support the "xecute" command for running arbitrary code strings, which is particularly risky if you ever use user inputs as parameters in the strings you try to xecute.
Also, I primarily want knowledgeable developers working on my healthcare and financial systems, not the newer ones who would somehow be incapable of using a first-class function.
> wicked fast at reads and writes. Great for a fast key-value store.
This isn't a differentiating feature.
> you're very unlikely to find a bug because it's been in production so long. This makes it good for mission-critical applications like healthcare and finance.
What happens when you need to change the code? Do you wait a long time until bugs are no longer found?