Every business has a feedback loop, and for this particular problem it is inverted: the people who could explain it are gone, and the people who remain are the ones for whom nothing is wrong.
Why drift is silent
Complaint is a sign of engagement. A customer who complains still expects the relationship to continue. A customer who has quietly decided you are no longer the obvious choice has nothing to gain from a conversation about it, and usually likes the people involved enough to avoid one.
So the exit is polite, gradual, and unexplained. Frequently there is no exit at all: renewal simply becomes a conversation rather than a formality, and then the conversation goes elsewhere.
Who does give feedback
The customers who stayed. By definition these are the ones for whom the current offer still fits, which makes them the least informative group about whether it has stopped fitting anyone else. Survey them and the answer will be reassuring and true.
This is why satisfaction scores can rise while share falls, and why that combination is a warning rather than a paradox.
The prospects who never arrived
The larger and more invisible group. People who considered the business and did not enquire leave no trace at all, and they are the population where language drift and expectation inflation do their damage. No business has a channel for hearing from them.
What partially substitutes
Lost-deal conversations, done badly on purpose. Not a formal debrief with the decision maker, which produces the polite answer. A short, genuinely curious conversation with whoever was actually comparing options.
Watching what is described, not what is said to you. How the business is characterised where it has no involvement is closer to the honest reading than anything a customer will tell you directly.
Asking new customers what nearly stopped them. The one question that reliably surfaces the objection that stopped everyone else.
The uncomfortable implication
A business cannot rely on being told. If the intention is to catch relevance decay before revenue does, the information has to be gone and got, and the places it lives are all outside the customer relationships that currently exist.