Relevance decay is a real mechanism and it is also a convenient one, because it explains a decline without anyone being at fault. That makes it worth holding to a standard.
The two diagnoses
Decay: the offer is still good and the market has moved around it. The response is realignment, and it is mostly about how the business is described, evidenced and found.
Execution: the offer, the delivery, the pricing or the process has a problem. The response is fixing that, and no amount of realignment helps.
Applying the wrong treatment produces a year of work that cannot move the number, and both errors happen routinely.
The four questions that separate them
Did perception measures move before revenue did? Decay shows up upstream first. An execution problem frequently shows in retention and satisfaction before it shows in acquisition.
Are existing customers still happy? High satisfaction with falling acquisition points toward decay. Falling satisfaction points toward execution.
Is the loss to a specific competitor or to nobody? Losing to a named competitor usually means execution or offer. Losing to no one, with deals stalling and prospects going quiet, is closer to decay.
Does the business appear at all where consideration now happens? Absence is a decay finding. Being present and not chosen is not.
The honest possibility
Frequently both. A business can be slightly misaligned and slightly worse at delivery, and the two compound: harder acquisition raises pressure, pressure degrades delivery, and degraded delivery makes acquisition harder still.
Where both are present, execution is fixed first. Realignment that brings more people to a weak experience accelerates the damage rather than repairing it.
The standard to hold it to
Decay is only a real explanation if it made a prediction before the fact. If perception was measured, held steady, and revenue fell anyway, decay was not the cause and saying so is the test that keeps the concept useful rather than comfortable.