Its not an argument, you're recycling an old favourite re-Berbatov which is rubbish.
Shevchenko was never anything except average speed.
He made his reputation in Italy due to his ability to put the ball into the back of the net once he was on the ball and his ability to finish chances created for him.
When Berbatov loses his pace it'll effect him. He'll play a lot like he plays now, only slower and winning even less loose balls, caught when going through more often, slower to create space for himself, etc. he wont really change his game he'll just become a lot less effective with it. Because he already plays the game of an old player.
Villa will still score goals when his pace goes. He'll adapt his game more to rely more on letting the ball do the running for him and making more use of his passing game more along the lines of Giggs. But we arent talking about a player with no vision and limited ability to play a pass. He'll have to adapt his game more than someone like Berbatov, but losing his pace will impact Berbatov's game as much as any other player because he needs every last bit of it to provide certain aspects of his current game.
Villa losing his pace, he'll still be fast enough to make runs through the middle, to create space quickly and to compete for every ball. He'll just do it at Berbatov's current pace.