"The Mail app in 10.6.8 was the last proper email app that worked really well and was visually good to use and had good features."
I can't exactly pinpoint it, but in 10.6.8, Mail was already crippled to a degree. I think it started in 10.6.6, that Mail started checking all accounts at once, instead in sequence. This was and is still important to me on a slow satellite internet connection. If 10 email accounts get accessed at the click of a button my connection peters out.
There is a load of other things wrong with the new Mail [automized account setup and losing sent emails for example and that one can't drag/restore email accounts by dragging them into Mail].
I completely agree with your sentiment regarding change for changes sake. Of course this all has to do with the "iOSiffication" of the Mac OS and pandering to the switchers. If one never used a Mac before, Mail looks quite alright. But I'm with Apple since the Portable and System 6 and was never more frustrated about Apple's direction than today.
I have seven Mac users around me and five use different email application, for exactly the reason you state and others. I am so frustrated about Mavericks, that I am gladly stuck on OS 10.6.8. And if I could, I probably would go back to 10.6.5. This also has prevented me from upgrading to a newer Mac than the 2008 MBP I'm using, which is getting a little slow when using Aperture.
By the way I have tried Mavericks twice. It was running fast from an SSD, but there were so many small changes to the Finder and other UI changes that totally ticked me off, that I could not get used to it.
And frankly, if I have to ask my boys several times a day, how to do this or where did this go, I feel embarrassed as a long time Mac user who taught them at one time the inner workings of the Mac. If I need a Bible to find my way around Mavericks, then I have either lost my cognition or Apple lost their founding principles of how to design an easy to understand and logic UI for their current OS.