A microwave oven's power diminishes over time due to the magnetron tube.
A Magnetron is a "vacuum tube".
One limit on vacuum tube lifetime is cathode emissivity - the ability to provide electrons for the 'tube' to 'modulate'. Decay mechanisms can be complex but the first approximation relates to the availability of materials that liberate electrons and the action of trace gases on the cathode surface.
[The material is usually not "used up" over the tube's lifetime but its efficacy may decline].
This is usually the main culprit on an older model.
The typical magnetron tube has an average life of 2,000 hours of operation.
''Some factors that can diminish the life of a magnetron are:''
''1) no-load operation''
''2) operating with too much metal in the cooking cavity,''
''3) line voltage consistently too low or too high''
''4) improper phasing, ...''
''5) continual operation at the upper limit of its heat tolerance due to inadequate air circulation,''
''6) obstruction in the wave-guide,''
''7) failed stirrer operation.''