I know of and have seen a lot of very important founders and CEOs who still all swear by Superhuman.
Once they (investors) have realized that there is no growth or anything to extract, or a lack of further fundraising, then it is written off and they move on.
Email is a funny thing. The UI (a list of subjects, senders, and dates) has barely changed in decades. And even now when AI is being added, the inbox UI remains the same. I'm not sure what I'd change but I find it surprising how little innovation there has been in that area.
As nice as this product is, I still wish Google would bring back Inbox. They created something really great and scrapped it for no good reason.
Edit: according to the pricing page, Notion AI can organize your inbox if you have the Notion AI add-on. Not quite sure if you can do this on free to some amount.
This is very surprising, actually because many people in the B2B sphere don't use Gmail for their work email.