According to the talk I've seen around at news and interview sites, mangakas are actually more conservative with their works than the publishers that own them. Licenses do revert to the mangaka once and if publishers give them up, but, apparently, many mangaka are reluctant to allow their works to be sold in digital formats.
For example, a 2009 article says outright: "As a rule mangaka are very reluctant to give up their works or give permission to use copyrights." http://factsanddetails.com/japan.php?it ... bcatid=135
And I see that kind of comment in other articles about publishing manga often.