By 2003, the only players from the Parcells era left on defense were McGinest, Law, Ted Johnson (who was only a run down player by this point) and Bruschi. And that was the best Patriots' defense in the team's history. Parcells didn't give Belichick Rodney Harrison, Rosevelt Colvin, Richard Seymour, Tyrone Poole, Roman Phifer, Mike Vrabel, Ted Washington, Eugene Wilson, Bobby Hamilton, etc.
And don't forget that McGinest was broken down in 2000 when Belichick got here. Willie was having back issues and was not playing at a high level. In fact after the 2000 season people debated whether Belichick would let him or Slade go with a lot of people wanting to keep Slade over McGinest. Belichick helped to revive his career by making him a 3-4 DE and having him play upright rather than his hand in the dirt.
And we are talking about Belichick the head coach. The head coach plays the players he is given by the GM. So who cares where he got the players. He created three great defenses in 2001, 2003, and 2004.