Additionally, the devs are working on getting the full release out (performance tweaks and bugfixing). Once that's done, they'll be able to deal with design.
Your idea isn't as simple as it seems: randomizing the players is really easy, but implementing this with a proper design alongside existing lobbies is far easier said than done.
In terms of design, there's work being done on making a proper mix system with rankings and everything. If we're going to go through the trouble of having higher class lobbies, might as well do it right.
Additionally, it's not clear simply randomizing will make that big a difference. It seems to me that people are more worried with finding the class they want to play than trying to stack teams. I think this only makes a significant difference for the medic, since he always wants to go on the team with the better heavy/demo and he usually has a chance to pick.