I do think that we might be getting to the point where increased distance will reduce enjoyment.
A "long drive" is a relative concept to some degree, but not totally. I wouldn't have fun playing 100 yard par 5's with a nerf golf ball no matter how often I outdrove my opponents. By the same token, however, I think if 400 yard drives became the norm, I would not enjoy the game as much, even if courses were extended so as to ensure continued challenge. My rationale is two-fold: (1) I would not be able to see the flight of the ball, including the bounce and roll, nearly as well. This would be reducing a great pleasure of hitting a good shot. Just as we wouldn't want a course with nothing but blind shots, don't we usually want to have the pleasure of seeing our shot bound its way down the fairway? (2) The short game shots would be totally incongruent. Wouldn't golf be silly if we were hitting our wedges 200 yards? Our short pitches and chips would become a smaller proportion of a full shot, which might end up seeming a little bit silly.
Indeed, I already have trouble seeing my driver and long/mid iron shots finish nowadays. I'd gladly go back to my Titleist Professional and 6900 yard tracks as the norm.