This is one of the most common and traditional shining exemplars of awful code. Absolutely nightmarish stuff. Such code can only be written by someone who has absolutely no idea what programming even is. They looked at a problem and said: "You know what this problem needs? It needs a much worse programming language that I just invented, jammed into a config file." So the code was tremendously awful to begin with, and you threw it out. Good for you.
> I said look, we're SWEs, we know how to edit code.
Absolutely right: this is why we use programming languages to program, and not config files. Again illustrating that you never threw away good code; you threw away the trash.
> I'm replacing this with something centered around an if-else tree that just does exactly what we need right now and can be modified later, with little attention paid to quality. I'm not even gonna bother splitting the codebase into multiple files cause someone is probably gonna have a strong opinion about organization; that person can have fun doing that. So far that's been modified a lot in the past 3 years with ease, it's gotten the job done, and anything more structured would've broken during that time.
A chain of if-elses is probably not paragon-level code, but definitely an improvement over what you described as coming before. My overall impression is that nobody has really taken the time to sit down and really dig deep into the problem, extract out the invariants, model things mathematically, identify the important operations you need to perform, develop useful data structures to support those operations, etc. My feeling is that if it was actually set up well -- really well -- you'd look back on your current situation and cringe.
It's statements like "anything more structured would've broken during that time" that make me think you really just haven't been fortunate enough to even experience what good code actually looks like. You can't imagine the situation being better. But it can.
Does it need to be? Maybe not. Either way, this is still a story of replacing awful code with better code.