The notion that you design a fighter with one one type of aerial opponent in mind just doesn't make sense to me, surely you build them to fight with, and beat, any opponent. I remember talking with an F3 pilot and he remarked that they never wanted to see an Su-27 - ever. Hardly inspiring.
But at the time the ADV requirement was set the notion that the Backfires could be escorted by a long range agile fighter such as the Flanker was inconceivable. Any fighter opposition was expected to be either the MiG-25 or the Tu-28/128, neither of which could be remotely considered as agile.