The cost of perfect SEO exceeds what ranking is worth the moment the marginal cost of more polish outruns the realistic value of the ranking it might buy. That line shows up in three places. The first is polishing past diminishing returns on a page or term whose realistic ranking value is modest. If the best plausible outcome is a small amount of traffic with limited value, then hours spent perfecting that page are hours spent buying very little, and the work stops paying long before the page is flawless. The ceiling on the term, not the perfectibility of the page, sets what the effort is worth.
The second place is opportunity cost. Even when a page could be polished further with some real benefit, the same effort often earns more somewhere else, which means continuing to perfect it is quietly the wrong call. The hours have an alternative use, and if that alternative returns more, perfection on this page is a loss measured against what you gave up. The third place is when “good enough to rank and serve” is already achieved. Once a page satisfies the query and competes, additional refinement is largely vanity, polish that makes you feel thorough without changing whether the page ranks or helps anyone.
This cuts against the instinct to always strive for perfect SEO, an instinct that confuses the means with the end. Ranking is not the goal; it is a means to value. A perfectly optimized page that earns little is worth less than a good-enough page that earns plenty and freed your time for the next one. The pivot is marginal-perfection-cost versus ranking-value, and perfect is the enemy of worthwhile. The discipline is to stop when the next increment costs more than it returns, not when the page can no longer be improved.
To find the line, ask what the ranking is realistically worth, whether the next increment of effort would earn more elsewhere, and whether the page is already good enough to rank and serve. When the answer says the marginal polish costs more than it returns, stop. Take the effort to a page where it pays. Stop at good-enough-to-rank-and-serve, and let perfect go.