Once a publisher has any book, for any reason, in digital form I don't see why they'd ever want to delete it. Not because keeping it is their "responsibility" but just because it might come in handy someday and it's not like it should take much work, or space, to keep it.
If digital formats change I assume? they'd have a process set up to convert current stuff so why not convert everything. There are probably people here on Mobileread who maintain Calibre libraries of more books that most single publishers have ever published.
