Speaking as a Jets fan watching the team from the outside, that's exactly the problem. When the Pats were signing guys like Pleasant, Phifer, Hamilton, Otis Smith, etc. - guys who were average to below average starters - they were signing them to be complementary guys to a defensive core that included Richard Seymour, Ty Law, Lawyer Milloy (replaced by Harrison), Tedy Bruschi, Mike Vrable, a young Ty Warren (in 2003 & forward), a young Vince Wilfork (2004 and forward), a young Asante Samuel (2003 and forward) . . . guys who were anywhere from "above average starter" to "perennial pro bowler"
Now, you're signing the same type of player, but to join a unit that lacks that type of top end talent. Seriously, your defense has only 3 players I'd consider above average starters at their positions - Wilfork, Mayo, and McCourty, though each is significantly more than just "above average". You can get away with "plug the gap, hold the fort" veterans when the players surrounding them are excellent. You cannot do that when the players surrounding them are equally mediocre. A team of average to below average starters will deliver average to below average performance.
That Belichick is taking the same approach to free agency he did in the early 2000s, at a time when the Pats have significantly less talent in-house going into free agency than they did during those years, is an indictment of Belichick as a GM, not a defense.