I don't think that's necessarily true. Larry Bird was a lousy defender, but the Celtics were still able to play good team defense. You can find other examples too.
I disagree with the premise that title-contending teams evolve over 8-10 years based on a "culture." The only possible example you can point to over the last 30 years is the Pistons. Titles are won by players, by the Jordans and Duncans and O'Neals and Bryants. The Suns don't have the players to compete for a title now, but that's not in Nash's lap. Dumping him early isn't going to bring in elite talent any faster.
As we've seen the last two games, the Suns are capable of playing good defense even when Nash is on the roster. Of course, it helps when he's on the bench
, but just having him around doesn't necessarily infect everyone else with bad defense.
The Nash Suns are the most efficient offensive team in the history of the league. If you expect to get higher-percentage shots by kicking him to the curb, you're in for a bitter disappointment.