Rune knight will be a tank, RDM was never intended to be a tank, althought it in some specific situations could act like one. In the current game, you do not invite RDMs if you need a tank, so this job doesn't really take much away from RDM.

In related news, they are planning on possibly revamping the entire enfeeble system and make it better. That could mean RDM can have fun with their enfeebles actually working on important shit again.