I agree that labels/tags are flexible, allow orthogonal organization, etc. but right now I have a simple inbox with different newsletter and it's very clear where they come from so I only need a 1-dimensional sorting.
And in fact I do love Vivaldi Desktop Mail's Mailing Lists view (no need to do anything, works out of the box, can even rename for more readable name), Filter view (good to fuse multiple sender email addresses into one view as some websites have support@, forum@, etc.)
It's just that I sometimes check my emails with Thunderbird (and on Android I have no choice as I see no Vivaldi Mail app, and I don't want to install too many apps so I'm sticking to FairEmail for now).
Thunderbird supports folders and labels as "tags", but I'd use those with the standard values "Important", "Work", "Personal"... I'm already creating new labels on Gmail for each type of mailing list and it's cumbersome, I wouldn't do it again.
So being able to convert Filter views to folders would allow an extra degree of compatibility with email clients that don't support Filters yet (it seems quite new so it may take some time to be added).
That said my Vivaldi email doesn't have critical emails at the moment and therefore I don't need to check it on mobile, and I'm happy to just use Vivaldi Mail on desktop for now. Coming from a programming background I also appreciate the concept of read-only filter views (like views in Python and more recently C++), and I'm eager to see them land in other email clients.
In the meantime I understand that Vivaldi Technologies prefers bringing new features breaking compatibility to get more adoption, but then they'll have to think about releasing a lightweight Vivaldi mail app on mobile (not having to install yet another fully-fledged browser just for that).
Apparently I can use Vivaldi Mail as a generic email client too, so I may try it with Gmail and see if it can sort all my newsletters per sender.