Obviously, one loop in Python won't hurt performance. But reality is that people will try to build SPA on this technology. They will port Redux, that uses immutable objects and recreates whole graph of objects on every event. They will try to move SQLAlchemy and Django into the browser. Isn't it cool (for inexperienced developer), you just import a script from CDN and can write Django code? And this will run on a 10-megabytes interpreter in WASM.

Of course if you are writing an internal app and can provide M2 MacBook to every employee, then it is totally fine. But if you are writing applications for wide audience, it is a different thing.

I remember that one of early users of Vkontakte (a Russian clone of Facebook) was impressed that the site was loading fast on his old computer. As I remember, its JS code was written in vanilla JS without libraries like jQuery. Today the hardware is better, but if you will run SQLAlchemy in a browser to save development cost, your site's loading time won't impress users.