Hard disagree, outside of the addition of LFD (right at the tail-end of the expansion) the game was at it’s peak during WotLK and kept on growing.
There were some hiccups sure, like the gearscore addon, DKs being OP and the aforementioned addition of LFD but no expansion was perfect and it’s positives far outweighed the negatives.
Cata was when the game really started going downhill, that was when the homogenization of classes began, Heroics became a joke and the over-abundance of conveniences and QoL changes started stacking up.
On top of that we had a string of half-baked content patches culminating in quite possibly the worst end-game raid ever since alongside LFR (which I personally believe was far more damaging than LFD).