If you flip the setting, that doesn't solve a problem of a user installing the browser for the first time, installing an extension like Stylish †, going to a porn site in private mode and getting their private browsing history leaked to the extension operator together with their regular history. Who thought that is a good idea? :)

† Stylish is a … special case [0]. But see also hypothes.is or any other extension that relies on a "user-created content store".

[0] https://robertheaton.com/2018/07/02/stylish-browser-extensio...