[Calhoun720]

Youre assuming there's no other sealing jutsu in existence that can do that. The bodies are just a medium for the soul to interact with the physical world. So sealing options are open. We havent been exposed to but a few seals in the series. Likely Shiki Fuujin is one of the most powerful as it contracts with the death god itself. This would allow The third to prevent their use (the First and Second) by Oro even after he dies which is part of the point of resorting to it.

He only had so much chakra left. Yes he used clones but under these circumstances the contract extends to the clones too. When Naruto makes mass clones they have to form their own rasengan from the dispersed chakra in each one. They dont just get one because the real Naruto makes one.

The Third knew enough jutsu to know of other sealing options. This seal makes it permanent even after he dies, and he had just enough chakra to do it. Fortunately, the First, Second, and Third shouldnt be able to become a part of the E.T. army.