Well for starters, I am not sure where you are going with the Coughlin example. Coughlin was a two time SB winning coach, who also was successful in Jax and in college. So letting him go, given his track record of success, was a risk. Garrett doesn't even being to sniff the resume of Coughlin, so firing him carries way less risk because he's done zero to date. So I am not sure this parallel you are trying to make, even begins to make sense.
As for the Chargers letting Schottenheimer go............ the year after they fired him, they were in the AFC Champ game. So in one sense, yes, they actually did improve after firing a highly successful coach (at least for that season, Turner kind of fizzled out after that).
And yes, you can be "mediocre" when you win a few divisions but spit the bit in the playoffs. We are lumped in with a bunch of other teams that win a couple of division crowns here or there, get bounced from the playoffs and slink off into the sunset. That's mediocre. It's not bad like the teams who don't make the playoffs but it's not good like the teams that win divisions, then make real noise in the playoffs. We are right in the middle, the teams that spit out a few good seasons here or there but are largely non-factors in the postseason.