Related, did you read Kenny Clark's comments yesterday about the 49ers' game? He tried hard to avoid it but he basically said "yeah, we've done nothing to improve the D." It's a fact: where's the optimism about stopping the run when it counts?
We don't need Kenny Clark to tell us that.
Martinez out, Kirksey in might or might not be an upgrade. Kirksey's injury history could certainly backfire. 5th. and 7th. rounders for the defense in the draft. Tramon Williams has not been resigned, a guy who had a nice season last year in case nobody noticed. With camp having started I would not expect him back unless a starter gets injured leaving the job to the unproven Sullivan whose notable plays came out of zone coverage.
There's nothing one can point to as an evident upgrade besides favorable projections which is really just "hope". It's probably worth noting that the injury situation last season was uncommonly favorable other than Greene. Reliable depth at OLB and CB are both down a player.
You really have to project steps up or breakouts from some combination of guys like Gary, Burks, Jackson, Greene, Sullivan, Martin or TBD.