I think you and the original mentioner of "hikkikomori" might be missing the actual social causes of it, it's not purely a "got no work to do, gonna just lounge about eating noodles and watching anime all day". It happens in a very, very rigid/conservative society, almost as a rejection of it.

If it were purely lack of work -> become recluse, we'd see this with 100% of retirees everywhere, but we don't. I'm not saying it's 0%, but I've known plenty of retirees with hobbies and socialization.