There are too many legendary and mythical Pokémon. It would be beneficial to retcon some of them out of existence so as to make the legendary/mythical status feel more special and prestigious again.

The legendary and mythical Pokémon that should remain after the purge are as follows:
>Mewtwo
>Mew
>Raikou
>Entei
>Suicune
>Ho-Oh
>Jirachi

That is all.