Ohtani is not going to continue to pitch and hit for the rest of his career. If you sign him for 10 years, he will probably become either a full time batter or pitcher and in like 3 years or so. Right now, he is just a DH as a hitter so you don't pay a DH 500 million. If he decides to just pitch, again, he is not worth 500 million. Maybe Ohtani does continue to be 2 way player at a high level for the next 8-10 years but it's never been done before so I highly doubt it happens. I am not sure how long he can continue to be this good doing 2 things.