I think this is a gross overreaction. The 49ers’ defense didn’t flounder because Saleh left; it regressed in the last couple years only because the DC was never allowed to remain in place to grow the unit.
Saleh left—Ryans stepped in, the defense was excellent.
Ryans left after one year—Wilks was hired. Defense was great, but Wilks had issues fitting in philosophically with the organization.
Wilks fired after one year—Sorensen promoted, was in way over his head, cut loose after one year, replaced by Saleh.
The 49ers’ defense (apart from time under Sorensen, and this year hit by injuries), has been great since 2019.
Shanahan isn’t any more “lost” than any other coach trying to replace an excellent DC three times (Saleh, Ryans, and now Saleh again) in a five year span.
Niners defense will return to the upper tier with any of several new DCs. The key is getting that guy to stay.