As for the need for agility - your adversary has a vote. If the opposition intends to fight into and through the merge, you will need (a) pre-merge dominance so he never gets there, (b) a magic all-aspect weapon that defeats him or his weapon (think DEW) or (c) competitive agility so that he doesn't control the engagement.
JSF people have claimed all three - stealth, EO-DAS/HOBS and comparable maneuverability. Unfortunately none are in the least valid.