Your claim that service workers are more idiomatic than background pages ignores months of debate and examples to the contrary collected by the w3c web extensions group. [0]

This could be an area where MV3 walks itself back. One proposal creates an Extension Worker which is a new concept like a Service Worker but better matches the use cases for background scripts. Another simply puts background pages back, but behind a permission.

[0] https://github.com/w3c/webextensions/labels/topic%3A%20servi...