You don't need great cornerbacks or even a deep depth if you have a dominant pass rush or solid long time veterans up front who educate those guys to a high level of performance.
Yeah,our cornerbacks were never a strong suit after Law left but there were always guys up front making plays to help the secondary,Bruschi with so many big plays,McGinest coming up with sacks and Seymour overpowering his man along with Vrabel who was a very smart and savvy OLB.
Those guys overshadowed weak cornerback play,when they all left the secondary was left to do without that help....and it's hard to regain those types of guys......man I miss that bunch.