It makes sense though. He was dominating for so many years after L's death to the point where his tools weren't being tested, he started to be more laid back, as there was no real threat to him anymore. The mistakes he made would absolutely not have happened if L was still alive because every little detail would have been considered and simulated multiple times, there would be almost no loop holes.
Having no competition can deteriorate your skills. In that sense his mistakes and the ending of Death Note was perfect.