You say Bruschi and Seymore played on a great defense. Yup. They were the ones making it great. How many people will ever say that about Urlacher?
A lot?
Oh yes. Urlacher led many legendary defenses. Those vaunted Bears defenses were a terror of their time.
The Urlacher Bears have twice led the NFL in points allowed. The Pats have led the league once, and have been second twice.
I wouldn't necessarily call the Pats a "legendary" defense, just a very good one. The thing they've had that has separated them from Urlacher's Bears is an excellent (and at times legendary) offense. Since 2001, the Pats have had a top-ten offense in all but one season, and have led the league in scoring twice. Urlacher's teams have only finished in the top ten in offense once, finishing 21st or worst seven times.
Now, I know that in your world, Urlacher should have been an Iron Man, playing QB, RB, OL, kicker, punter, returner, and linebacker to lead his team to multiple titles. For the rest of us, it seems fair to say that Urlacher was handicapped by a terrible offense. The one (and only) year he was handed a top ten offense, he ended up in the Super Bowl.