Why do people say heimerdinger falls off?

Eventually the enemy team will HAVE to engage whether it's a seige at inhib turret or baron or dragon. In that case heimerdinger will excel at defending a position. He can just save turret kits anyway and plop it down if you engage elsewhere. . . His dps is good and he has a (conditional) stun but that's good enough as cc if he's good. Oh and don't forget early game he will probably get the first 2 turrets pretty quickly and that could snowball in global gold and you need the jungler to fail gank in order to try to save the lane. Heimerdinger other lanes have less pressure on them.