You make a very interesting point.
However your point begs the question:
Why should DA be responsible for changing your mailboxes from mbox to Maildir?
Since their mail system works.
It would be a lot easier for me to say "DA should fix it", except that from DA's point of view, I don't see what's broken.
Perhaps you don't know the history.
DA was originally sold with only POP capability. POP capability was traditionally supplied using mbox format (files, not directories), and so that's what JBMC used.
I can't speak for the specific reason JBMC added IMAP, but I know it was right after I wrote them that I couldn't use DA for my clients unless they could support IMAP, because the Cobalt RaQs my clients were currently on allowed IMAP and many of my clients were already using it.
At that point the easiest way to support IMAP was to use a server that supported mbox, and that's what they added.
It worked for me. It still works for me.
Perhaps because we understand the difference between email as provided by a hosting company and email as provided by a specialty email provider, and we offer both.
Note that Maildir isn't a total panacea and there are plenty of people it won't work for:
Maildir has each email in a separate file. Large drives have a large minimum file allocation and very small emails can rapidly use a lot of space. And each file uses at least one inode; at some point any unix/linux system will run out of inodes. Using thousands of inodes for a mailbox is not necessarily a good use of resources.
Personally I can see where many of our servers will stay on mbox, while some might be moved to Maildir.
Jeff