I'm not one to think advanced stats are end all be all, but they show significant improvement from Week 1 to Week 17, and were it not for the two blowouts in Week 2 and 3, the Niners D would be Top 20 overall. Obviously not phenomenal but good considering the new scheme, the lack of pass rush without the drunk, and so many 1st or 2nd year guys.
Plus Niners D stats at home for points allowed was 2nd fewest and yards per play 8th fewest in the league.
I'd say that's at least worth one more year for consistency sake. Don't forget our O led the league in 3 and outs and was bottom 3 for time of possession. That doesn't help.