Shanahan's big mistake was wanting to be a CEO of football operations essentially.
He doesn't know defense, yet he wants to be the one to pick personnel on that side of the ball.
IMO, the league is not phased by small oline, zone blocking scheme anymore. If you notice, shanahan offenses since the tail end of Denver days and with skins are atrocious on 3rd down conversions. The 3rd and 1 and 3rd and 2 situations where they get stopped on a regular basis is embarrassing