Killing at 0% against competent opponents will not consistently happen over time. Meta Knight also has specific weaknesses to his design that keep him from being a genuine top tier. His reward game mostly relies on the requirement that the oppnent fails to DI the hit confirm. One of the reasons why Captain Falcon is bad is because he gets very little off of his down throw if DI away is used. Meta Knight suffers a similar problem. He also is very vulnerable on the ledge. While he can always return to the stage, get on the stage can be challenging against characters like Zero Suit, Sheik, Villager, R.O.B. and any character with good ledge coverage. His neutral, while not bad when playing against reasonable characters, is very restricting against characters who have exceptional neutral options. His reward game does not mean anything if you can never win neutral, or if your opponent can DI your confirms upon losing neutral.
He's great, but once players properly optimize his punishes, players are going to be forced to learn how to avoid dying to his lethal combos and that is when he will likely settle into his 'proper' tier placement.