Since when is Python the “default choice” for server side dev?
Really, I think the main driver for isomorphic Python web libraries like this isn't general web apps (though they'll get used for that, too), but the putting frontends on data science, ML, etc., stuff where Python is the dominant tooling language.
Jupyter and Shiny seem adequate for that.
Which is why I'm a bit surprised that RPC/APIs still have as much overhead as they do, especially around types.
People use either Java, PHP or NodeJs.
By and large "people" are not "displeased" by Python. But every language has a set of diehards, people who dedicated decades to this or that and resent adopting anything else. They will complain no matter what. Python is displacing a lot of other platforms in its continued rise, so the noise keeps going up.
In addition, there are groups of nerds that love to formalize absolutely everything in their code, development speed be damned; and people who will optimize every line they write to an inch of their life (again, dev speed be damned). Both those camps are often uneasy in an ecosystem built on duck-typing, "practicality beats purity", and an overall approach favouring development speed over machine speed.