Regardless of if his body wasn't what it used to be, one thing I know for sure is this;

Byrd is not a single high safety, he was a ball hawk in Buffalo because they let him do what Ed Reed did in baltimore, roam the secondary and undercut crossing routes. Watch his Buffalo highlights, he plays 10-15 yards from the LOS.

Payton had him playing 25 yards downfield. It's not entirely Byrd's fault, he chased the money like an aging player would and Payton was dumb enough to pay someone 56 mil to use him entirely incorrectly.