Flawed? Sure, although every language has its flaws, some more than others. Deeply flawed? Not really. Obsolete? Lol, what. It's top 3 most popular languages.
Python's performance problems are unfixable without wholesale redesign of the language and standard library. Its package
management is a bad joke. The developers give no indication that they have any intention to fix those problems, rather than keep stuffing more unnecessary and badly thought-out features into it.
Obsolescence should be gauged by how modern the design of a language is, rather than its popularity. By that measure, Python is manifestly obsolete, and has been even when it debuted.
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...