There are other use cases for containers besides third party cookie isolation. If you want to have two separate sessions for a site, you'd still need containers.
Containers makes this a breeze.
In addition, at least Firefox only has a single private session. So if I open a site in one private window and another in a different one, they're in the same session, sharing cookies etc. Not so with different containers.
I don't understand why fixing profiles isn't a priority, I find the usecase for them is completely different to containers which are awesome in a completely different way.