> I wish a browser offered the idea of opening windows/tabs with selectable incognito identities.

That's exactly what the Firefox Containers feature is.

It's not that easy to use, and extensions that improve the UX or automate container selection are kind of messy. But it does work. It's excellent for when you need to use multiple accounts on a service.

Agreed that is a pain. In fact if Google did it they'd likely be accused of making it less useful to purposely get people to not use it.