What if you're not subscribed? I don't usually care about the day to day of Linux kernel dev, but that one thread might be of interest to me. Actually, I might have something to contribute, but since I'm not subscribed the UX on that sucks too.

"Just use mutt" as a reply to "I want to read this one email thread" is rather missing the point. Plus the reality is most people neither use nor want mutt. Many people think the UX on mutt is horrible. Nice if it works for you, but it doesn't for many. So there's that.