The most counterintuitive of the four mechanisms, because it involves nothing going wrong. The proof was true, it is still true, and it has quietly stopped doing its job.
Old evidence reads worse than less evidence
A page of case studies dated four years ago does not say that a firm has a track record. It says that nothing worth writing up has happened since. That reading is usually unfair and it is the one a stranger arrives at, because the alternative explanations require knowing the business.
A single recent, specific result outperforms six impressive old ones, and businesses reliably believe the opposite.
The four things that age
Named clients who have moved on. A testimonial from someone no longer at the company is checkable and comes back empty.
Results in outdated conditions. A campaign result from a different market, a different platform, or different economics is not obviously transferable, and the reader has to do that work.
Credentials nobody recognises. Awards from bodies that folded, certifications for technologies that are gone.
A portfolio reflecting capability you have outgrown. The most common and the most damaging: the visible work is smaller than the work the firm now does, so the price it can ask is anchored to the old version.
Why it never gets fixed
Because updating proof requires asking clients for permission, which is uncomfortable, and because nothing signals that it is due. Nobody complains about old evidence. They simply weigh it less, silently, and nobody inside learns that it happened.
The specific check
Look at the date on every piece of proof the business publishes. If the newest is more than eighteen months old, evidence decay is already in progress regardless of how good the work has been since.
The connection to everything else
Evidence is also what independent sources cite. A firm with no recent verifiable results is a firm nobody has a reason to reference, which starves the slowest and most valuable signal it has. Evidence decay is upstream of a corroboration problem that will take years to repair.