YOU SAID:

whatever goes up, must come down.

INTO JAPANESE

何が上がるは、降りてくる必要があります。

BACK INTO ENGLISH

What goes up, must come down.

INTO JAPANESE

上がるものは、降りてくる必要があります。

BACK INTO ENGLISH

What goes up will come down.

INTO JAPANESE

上がるものは下がるでしょう。

BACK INTO ENGLISH

What goes up will come down.