This is so true. And bear in mind that Mike Shanahan was Parcells and Belichick's bete noire in the 1990s. They struggled with that system throughout the decade. Shanahan beat the Pats 34-8 in 1996 (when BB was running that D) and then 27-21 and 23-10 vs. the BP/BB-led Jets in 1998. He also beat the Pats under Parcells 37-3 in 1995. BB beat Shanahan in 1999 and 2000 (Jets first, Pats second), but Shanahan then beat BB in both 2001 and 2002. The Pats won 30-26 in 2003 and then Denver beat NE twice in 2005 (regular season and a beat-down in the playoffs) and in 2006. The scores kept getting closer through all of this, and it is very clear that BB invested a ton of energy in figuring out how to stop it. Finally, BB crushed Shanahan 41-7 in 2008 (the Cassel season). In his last game vs. BB (Washington in 2011), the Pats won 34-27.
Anyway, Shanahan's system was a thorn in BB's side for ages. He spent a LOT of time trying to master it.