How many boards have more than 100k elements? (I'm reasonably certain that they can handle more, but ....)

Your assumption that Python necessarily results in an "unmaintainable mess" is "interesting," that is, wrong. More to the point, unmaintainable messes can be written in any language, no matter how "modern."

Not only that, but it's irrelevant. When I was a young engineer, I pointed out that a given system wasn't scalable. The person in charge of the project said "Yes, but by the time that scalability matters, I won't even be managing the person whose problem that is."

As to whether their choice of Python is fatal, feel free to do it better in a "modern language."

If you're right, you'll do well and good. If you're wrong...

What? You're not going to step into the ring?

"reasonably" is out of place here.

> our assumption that Python necessarily results in an "unmaintainable mess" is "interesting," that is, wrong.

It's not assumption, it's a conclusion based on the state of Python's "ecosystem"

> As to whether their choice of Python is fatal, feel free to do it better in a "modern language."

Yeah, the only people who can judge food are cooks themselves.