I think it's possible to use tkinter to develop even modern apps which don't look primitive, there are many open source projects which have done that including the IDLE editor itself, a part of core python project. The only issue is that it sometimes becomes excruciatingly difficult or painful to even perform some very little non-trivial things (like customizing a data-grid with a different style or having multiple tabbed pages) which are quite a simple task in other toolkits as they're already available in the form of a widget or component.

If ever I decide to leave tkinter, I think a viable alternative I will try is free-pascal[1]. It's a more comprehensive and well-thought toolkit for creating GUIs, more importantly it is also open source and cross-platform. It's the FOSS version of Delphi IDE.

[1]: https://www.freepascal.org/