Yes, I know, that's exactly what I do, my DH mail is a an aggregating account collecting mail from all sorts of other email addresses, I need the aliases to send back replies from those accounts. Atmail is a disaster, and useful-feature free.
I don't think DH care if people go elsewhere. They will have thought about what they're doing, they're not stupid, and I can only think that they have decided to change direction, they don't want small businesses with all their demands, they probably want just non-techie users, who are used to dreadful email interfaces like gmail and the like, who will just suck up what they're given and not need the extras.
All companies end up like this eventually, they start off with good products, small and nippy, then expand and change out of all recognition, losing their original customers but planning on gaining a load of new ones who will expect less and gain them extra profits.
I wouldn't lose any time arguing with them, they're not going to listen, either sort out your own solution (20 webmail clients installed over a period of time perhaps) or find alternate hosting (which, as I have found to my cost, can be very painful too).