Other languages also have advantages in certain areas, e.g. Node.JS lets you reuse the same code on the server-side and client-side, something which is great for those wanting to right games that require the server to simulate them and the client to run them.
Even if the language is rarer, say C or C++, there are still many circumstances where it is better to use those than it is to use Python.
I do not foresee any language to be able to be easily usable in every use case, the generality itself would be a drawback for specific user case. There is a reason there are currently dozens of popular programming languages out there.
Also it's great for SSR.
Don't have to worry about if the join function has different signature per language.
Isomorphic JavaScript doesn't mean you spaghettify your front-end into your backend, that's an absurd thing to assume and assert is true. Yes, your front-end and backend can share npm modules, functions, and templates. This means your HTTP library is fetch or axios (basically, same API) rather than 2 different APIs). You don't have to write a jinga2 copy of your react modules for prerendering. You don't have to write your custom validation rule in 2 different languages. This is useful pretty often.