I didn't read the entire thread, but I did read what @Vlad said about email being one of the core features of Kagi. As for me, using email since 1994 and registering my first domain in 1995, I've been through the ringer with different email services, as well as self-hosting email with providers who hosted my domains.
For the past few years, I've been using Runbox, which provides a good alternative to other email services. It would take a lot to get me to change - especially because email is essential for my work as a freelancer, and switching email providers can be problematic - but I understand the importance of getting people to understand that Gmail is shit, and any free email is probably shit (exception for Apple, since they don't use ads, collect data, etc.)
I think a lot of Kagi users are in my situation, but then we're not the target demographic. We're early adopters of a new paradigm, and it's the second wave who could be sold by search and email together.