It depends, really. Nintendo needed to get a new console out there so it was time for them. Microsoft / Sony on the other hand... well, late 2013 sounds like a good launch date to me. I don't agree about there being a lot of untapped potential. We are definitely at a point where PS3 / 360 are becoming too weak for some games. Look at Skyrim on PS3 or compare the PC versions of multi platform titles to the console versions.
Late 2013 should be an ideal date to launch PS4 / Xbox 8 (or 720 or whatever). We don't need to buy those consoles right at launch, christmas 2014 should be the right time to make the jump for a lot of consumers (both consoles should receive their first price cuts around that time and the first titles that are truly optimized for the stronger hardware usually arrive a year after launch).
Late 2013 should also be the time when the Wii U really starts to get a flow of must-have titles (Mario Kart is a given for 2013 and there'll be more) so the other two shouldn't wait much longer than that. Else we'll have a PS2 vs GC / Xbox situation.
Edit: I forgot to mention the best thing about console twilight years - cheap software! The PS3 essentials line will grow over the next months (it's off to a good start already) and here's hope Microsoft will give us a €/$20 line of games as well.
