Defensively Chelsea have been under more pressure this season than really any since Cech came in, yet they are comftably the best defence in the league, which suggests several things. The offence/midfield is the worst its been for some time, the defence is under more pressure than any other year, yet they still aren't conceding...... some people would suggest the defence has been better than in recent years and the keeper aswell.
You have to factor in that in Cech's "best year" barely anyone got near Cech in the first place, this season Chelsea's midfield has just let far more attacks than normal get past them.
I don't think Cech is the best in the world but, I don't think theres a keeper out there that would change how good Chelsea are, several "sidegrade" options, probably 7-8 keepers who are as good, a bit better, a bit worse, but theres no one so much better Chelsea would have conceded so many fewer goals they'd be top or anything.
Cech is MORE than good enough for a title winning side, he's 29, has at least 5 years left, maybe a decade, probably somewhere in the middle.