I don't think that JMac is a direct replacement for Richards.
Richards' role was mostly used as an extra in-the-box guy. I'm guessing that JMac will backup deep safety, keeping everything in front of him, and that Harmon will back up Chung and be the 4th safety, in-the-box, when necessary.
Just to be clear, I'm saying not that the starting roles will change. DMac is still the primary deep safety, with Chung as primary SS, and with Harmon continuing as the nickel safety in either a deep-coverage role or up in the box, as the situation requires. If Chung gets hurt, Harmon is his replacement instead of Richards, and JMac takes a deep-coverage role in the nickel with DMac moving up closer to the line.
I don't think it's a direct replacement either, just that the more options we have amongst the safeties, corners, and linebackers, the less likely we'll a Richards type (ST player who shouldn't be seeing extended defensive snaps) forced to play.
Assuming Butler played (not trying to hash up that argument again, but just for comparison), in the Super Bowl we would have had:
CB:
Gilmore
Butler
Rowe
Bademosi
S:
McCourty
Chung
Harmon
Richards
Now to start this year, we have:
CB:
Gilmore
Rowe
J. Jones
JC Jackson
Keion Crossen
S:
D. McCourty
Chung
Harmon
J. McCourty
So at corner, Rowe is a step down from Butler as a CB2. Butler had a down year compared to 2016, but is still better overall than Rowe until Rowe can prove he can do it healthy for a full season. But I think J. Jones is a step UP from anyone we had in the slot last year. So a step down at one DB position, a step up at another. The key will be if Jackson and Crossen can develop, and both are better than Bademosi, then the depth will be better. Again, top talent takes a hit, but depth improves.
At S, it's basically the same, but J-Mc is a better defender than Richards (in theory). He may not play the exact role, but I'd feel more comfortable with him as the nickel or dime DB rather than Richards, regardless of what the overall philosophy is trying to accomplish.
If you want to be pessimistic (not you specifically, talking to the general audience now), you could wonder what will happen if Rowe gets hurt, none of the rookies contribute, the veterans at S get old, and Jason never "gets it". I prefer to be optimistic at the beginning of the season and see the ways we could improve, because that's more fun to me. I don't have a crippling fear of disappointment if I don't try to be "realistic" at every turn and prepare myself for the worst.