Overall, I feel that plugins making their custom UIs remains a necessary step if their features grow too much in complexity. I don’t like it either: the idea of feeling inside a new “system” when you open these pages.

But the truth is, as many said here before, WordPress UI System is not that consistent either, which is why the Site Editor still feels a bit like a plugin (aka that “Gutenberg part”). And even considering the consistency that exists today in Admin components, they are – lets admit – limited. That’s why people end up creating their on sidepanels, tabs, search inputs, collapses, etc. In that sense, I feel like the Design team is doing a gorgeous work of evolving the Gutenberg-side UI components. I just hope that those can be unified in the future with the Admin, in a way that plugin developers can take more advantage of it. That’s why I got really excited when I first saw this experiments going on (check the latest video):

https://make.wordpress.org/design/2022/06/13/thinking-through-the-wordpress-admin-experience/

This is a constant debate I have with my team in Tainacan because part of our “feature” is to have a dedicated UI, more thought for users. But in the end, I’m still dreaming of the day that we can be more integrated with the WordPress Admin.