Not wild about LONG followthru. We can agree about trying to hit thru 2.5 tennis balls, I think. What happens afterwards is less important. We don't need to wrap the hitting arm around the body all the way to the back fence, do we?
Just like a serve, after we hit the ball, we don't need to extend the followthru so the racket goes past behind the player, do we?
Stop the followthru sometime after hitting the ball, so we can recover and get ready for the next ball.