I've casually ragged on Richards' lack of contributions quite a bit, and I suspect that he may be in the "likely cut" category in TC this year (unless he makes an unusual leap in development during OTAs/Camp).
Richards played the same number of defensive snaps in 2016 as Ebner (18 all season), and played 13.7 ST snaps per game in which he was active (11 total games), which isn't "bad", but isn't "top level", either. For comparison, Grissom was active the same number of games (11), had fewer defensive snaps (11 total), but averaged 22.7 ST snaps in the games for which he was active (tops on the team).
While I agree that the coaches may see more in Richards than we fans are privy to, the fact that Chung played a killer number of snaps in the 2016 regular season (1207 between defense and ST, highest on the team) with virtually no relief (he missed only about 40 defensive snaps all season), would seem to suggest that there was no "safety" on the roster capable of giving him a break without making the secondary significantly more vulnerable.
It certainly seems possible that one of the current crop of UDFA rookies (or a street veteran yet to be signed) could beat out Richards, at least as a more legitimate 4th safety/ST contributor. And, it doesn't seem like spending a late draft pick on one would necessarily increased the Pats chance of finding a Richards replacement. Montae Nicholson was taken at #123 (Skins) a few picks before BB selected Wise at #131. The next safety taken after that was Nathan Gerry at #184 - nearly two rounds later, and one pick after the one BB traded for O'Shaughnessy.
In terms of comparing Richards to Chung on measurables ...
... Chung had nearly twice as many bench reps (25-13)
... Chung had a better long jump (119"-111")
... Richards had a better 3-cone (6.74 - 7.11 seconds)
There aren't really and meaningful differences between the two on other measurables (neither one showed particularly "elite" athleticism in pre-draft testing), and there's virtually no difference in size.
In terms of where he was drafted (last pick in the 2nd round), none of us fans have any way of knowing for a fact that there wasn't another team that would have taken Richards between the #64 and the Pats' next pick at #97, despite the overwhelming number of claims that "the Pats could have gotten Richards much later."