It was. And then that time came and went and after that it became harder and harder to justify its continued existence and development of applications.
I've worked on a moderately complex mumps system for a bank and the amount of arcane trickery to work around the limitations of the core MUMPS system were mind-boggling. The whole thing had to interact with other systems via trick comms links and the fact that it worked at all was a small miracle.
There are some other systems that are still in use even today that should have probably never been built in FoxPRO, Filemaker and other pieces of software of a certain age.
Companies that develop their own idiosyncratic dev systems are generally making a mistake. Developers who endeavor to master those systems are using energy and time to add a skill to their resume that has zero value on the open market, which is a bad career move.
Unless a tool gives you a major advantage you should never go near that. Your competitors will all happily use COTS, drop projects and adapt new tech while you will be stuck in the hole with your sunk cost and impossible to migrate toolchain. That sort of situation has sunk more than one company.
Folks who invest their careers in single-company toolchains tend not to have very remunerative or rewarding careers. It creates a monstrous imbalance between employer and employee, too -- all the "lifers" who stayed at TCK were being paid well below market, but couldn't command better elsewhere because their whole professional life had been spent on single-site tools.
That's MUMPS for you.