DDX works like Xtol on Delta 400. Those are the only two good developers for it in fact unlike previous generations of Delta 400. But I dislike liquid developers, specially ones without expiration dates and consumer can interpret.
Even if there were no patent issues and there may not be eventually I would doubt if Ilford could compete with Kodak's Xtol price and it would have to in order to make it worthwhile.
I am a recent convert to Xtol from DDX. So far in the 400 film range I'd say it competes very well quality-wise with DDX and price-wise there's no contest. Xtol wins hands down.
I feel the 5 liter solution is to much for me, I´m not developing that much film. Also I like using quite fresh developer and Fomadon Excel gives me that in the 1 liter package.