Why maneuverability and top speed are still issues in 5th generation air superiority fighter jets
Fighter jets are not bricks with powerful radar and long range missiles. BVR combat is an extremely complex affair conceptually, operationally and technologically. 


For BVR combat to be initiated, the enemy aircraft should first be identified and sorted as a threat. However, assuming that both have similar radars, the inverse square law ($S \ \propto \ \frac{1}{R^{2}}$) means that you are exposing yourself to the enemy before identifying him. Combat between two stealth aircraft (F-22 vs T-50) is not unlike two men with a gun and flashlight in a dark room, as someone noted. The first one to switch on the radar is actually at a disadvantage and against a highly trained enemy, possibly suicidal. Also, let's not forget SAM ARM missiles that can home on the aircraft radars.
Though the number of aircraft shot down by radar guided missiles (as opposed to IR missiles, which are usually fired WVR) have risen significantly both in numbers and also as a percentage (of total aircraft shot down), the probability of achieving a kill by a BVR missile is still uncomfortably low. For example, the following table shows the ratio of 'kills' by radar guided missiles post WWII.




Table from Promise and Reality: Beyond Visual Range (BVR) Air-To-Air Combat by Lt Col Patrick Higby, USAF


The high percentage of BVR kills in Gulf war is basically due to two factors- AWACS and IFF(Non-Cooperative Target Recognition (NCTR) system), which gave sufficient confidence to permit BVR shots. It is highly unlikely that such a superiority level and confidence is available in case of war between two peers.
Another thing is that barely any of the BVR shots are fired head on due to the high closing speeds. So in all probability, you have to maneuver yourself in a position where a tail (or an above) shot is possible. 
Also, if some missile is fired at you, speed and maneuverability are critical to carry out evasive maneuvers. 
Most of the combat aircraft use in present day has nothing to do about air superiority. They are mostly used as precision guided bomb trucks. Optimizing fighters for BVR combat sacrificing other parameters is, at the least, foolhardy.


So far, the most important contributors to BVR victories (in Gulf War, for example) are not due to some inherently superior technology, but due to human factors- pilot skill and opponent's weakness. It is potentially catastrophic to generalize the doctrine against a near-peer opponent.