alcatraz wrote:... outperform the aero frames and descend properly with disc brakes..
I won't repeat what some poster already excellently summarized above, just to comment on your original 2 points:
1) TT bikes outperform aero frames only because of TT base/extension setup. Cervelo or Felt themselves said (can't recall exactly, which of them) that their aero model (S5 or AR 2014+, respectively) performed equal to their TT bikes in the wind tunnel if steering setup were the same.
2) proper descending even on amateur level has very little to do with disc brakes. What matters is, the cornering skill of the rider plus bike "agility" (several "interconnected" parameters, such as wheelbase, chainstay length, fork characteristics), and, as pointed already out, on all of these any dedicated TT frame is severely lacking.