Cech's made plenty of mistakes tbh, hasn't ever been the best at keeping out one on one's which is what defence with a BFG the speed of a slug will leave you supsectable to. However his reactions are fantastic and obviously his height and reach are a big advantage, his shot stopping his second to none which are essentially the same traits as De Gea.
The seasons we had Luiz/Bosingwa/Cahill and Betrand in front of him we finished 6th in the league /parked the bus and became a unit to win the CL, and we're knocked out of the CL when we actually tried to play high lines and had poor defensive performances. Being a bit selective with your defensive choices as well - pretty sure you were successful when you had Keowne, a decent Kolo Toure, Campbell and Ashley Cole.
De Gea this season is a bit of an aberration. Never seen a keeper outperform the players he's had in front of him to anywhere near the level he has this season.