The Yahoo Mail interface is much cleaner than I remember, though I don't enjoy the purple theme, the loading times, and the persistent "Try Firefox" banner in the top right.
Plus my mailbox is full of spam from Yahoo itself, like weekly emails from Yahoo Movies.
We, non-Chrome users, hate persistent "try Google Chrome" banners on every Google property too.
When layoffs were announced Mail was spared and actually grew.
I don't know why it hasn't improved. It's not for lack of trying.
Ah, that's why Yahoo has gone to hell — emacs users don't stay with the company. You see, emacs has 8 different mail clients: rmail, gnus, mh-e, wanderlust, mew, vm, notmuch & mu4e — and no true emacs user (not to be confused with no true Scotsman …) would want to read email in anything but his favourite text editor.
I don't believe that encompasses Mail.