Sometime this decade, a lot of marketing leaders will have the same disorienting conversation: leads thinned out, organic traffic eroded, and no obvious cause. The truth is the ground shifted gradually and with plenty of warning. As AI answer engines replace click-based discovery — with Gartner projecting a 25% search-volume drop by 2026 and only 16% of brands currently tracking AI search — one capability increasingly separates the winners from the left-behind: the discipline of evidence. This piece explains why evidence-based marketing becomes the deciding competitive asset from 2026 to 2036, why the advantage compounds on a closing schedule, and what it takes to pull ahead rather than scramble to catch up.