It's a myth that one coach can completely seal the state border. Maybe in a one-year cycle it could happen, but too many forces at play working against the idea that a Missouri coach could land every in-state player he offers. For one, there's just too much competition. The SEC started to infiltrate the state once Mizzou joined the league. The Big Ten and Big 12 still recruit the state. Notre Dame still recruits the state. Most schools recruit more nationally than regionally compared to a decade ago, so there's far more competition than the old days where maybe Mizzou had to fight off Illinois, the Kansas schools, Nebraska and a couple more from neighboring states.
Second reason: Not everyone grows up wanting to stay home for college. No matter how great the new coach is, you're always going to have some players who want to explore other parts of the country, perhaps get farther away from home or have a connection to another school or state.
But all that being said, Drinkwitz is doing a fine job recruiting around the state, especially in the St. Louis area. Keep in mind - and he;ll learn this soon enough - it's a temperamental community. It doesn't take long for the good vibes to sour. Winning over the St. Louis region is never a one-time challenge for Mizzou football coaches. It's an ongoing process that has ebbs and flows.