This is not true, remember Adailius Thomas and Rosey Colvin were big FA acquisitions. The pats paid them tons of cash, and the results were very mixed. Bodden was an acquisition. Sparks etc. They have gone after free agents, and trades, they have not all worked out real well.
It is the draft, they had too many wasted high draft picks, and passed on too many good young players who are now superstars, either because they missed in the draft, or they traded out of their draft position to get more picks.
FA and trades are only suppose to fill in the gaps, you build your team from the draft. This defense has huge holes, they need an elite pass rusher desperately. They need play making OLB. They need better CBs, I ironically think they are fine in the middle, NT, MLB and safeties. But they are crap everywhere else.