Wikipedia:Notability does not degrade over time
Notability, defined on Wikipedia as being the subject of significant treatment in reliable sources, does not and cannot degrade over time, but can only remain steady or increase. It is a ratchet. Just as a bell once struck cannot be unrung, if a subject receives significant treatment in, for example, ten reliable sources published during one year, but is never again mentioned in any compliant source, it will always have those ten published sources upon which its notability is rooted; sources do not go away. Thus, notability cannot decrease, or degrade, over time. If only one new published source for the subject is published the following year, the subject would then have eleven established sources. While consensus may change, notability once established and demonstrated is permanent.
If this were not the case, ancient historical subjects, who are no longer with us, might be subject to arbitrary and frivolous deletion nominations based on mistaken assumptions about the ability of notability to degrade. Notability once conferred is perpetual, unless the thresholds for notability as defined in Wikipedia:Notability are changed or the assessment of notability was flawed.
Once sources exist they do not simply go away even if they become old, generally forgotten, superseded, or hard to find. The notability demonstrated by those sources persists.