I don't agree. Having front-end and backend in the same language is so convenient I would never go back to doing it the old way. I'd rather compile the frontend to WASM than introduce a mismatch.
I used to use Django and there were so many issues that arose from having to duplicate everything in JS and Python.
What about HTML? Are you writing HTML via JS - if not you're already writing multiple languages.
HTML is not a programming language. But yes, I don't write much HTML anymore.
The issue with mixing languages is that they have different data models, even simple things like strings and integers are different in Python and JS, and the differences only increase the more complex the objects get.
Sometimes I write some code and I realise that this code actually needs to execute on the client instead of the server (e.g. for performance) or the server instead of the client (e.g. for security). Or both. Using one language means that this is can be a relatively simple change, whereas using two different languages guarantees a painful rewrite, which can be infectious and lead to code duplication.