The story there is that Marissa Mayer wanted employees to use it internally, so it got heavily dogfooded and improved. (Yahoo employees were using Google docs and sheets for company use like password lists. lol.)
It's worked as your described above for a few years, but they claim to have made speed improvements recently (as it was unusable on low-end devices like netbooks.)
The problem with Yahoo Mail is that Mayer allowed a FISA order to install a kernel-level sniffing module on email servers, and I never heard if that was removed (interpret that as a no.) In addition, their network was pwned multiple times, so there's that too.
https://www.engadget.com/2016/10/08/reuters-yahoo-email-scan...
https://www.theregister.co.uk/2017/02/16/yahoo_forged_cookie...
Source: ex-Yahoo, offered role as technical lead of Mail.
In fact, except for (or maybe due to) the entirely blank pane on the right that presumably has a bunch of ads blocked, looks cleaner and more appealing than what gmail has become.