I cleaned my MAF & throttle body @ 65,000km (both looked very clean), I then bought a Durametric and checked the codes @ 69,000km and found code P0102 - Mass Air Sensor below limit value. The MAF was replaced (warranty) and I found a real improvement in mid-range engine response and a de-crease in fuel consumption.
So I suspect that the MAF failure is not just an on/off type breakdown but is a gradual degradation so you don't immdediately notice a falloff in performance....
That Durametric paid for itself first time out - what a great tool !