RAW (not RAI) argument that you can't force move creatures into an occupied square?
>You can’t willingly end a move in a space occupied by another creature.
Have we been reading too much, but not enough into this line?
If I push a creature onto another square I am actually moving them, therefore if I push them onto the square of another creature I am "willingly ending a move in a space occupied by another creature"
Note that the rules do not specify that you can't willingly end your move.
I wouldn't try to read this much into it usually, however the new D&d beyond drop more clearly indicates that pushing creatures into another space is not RAI.