But wouldn't there be big gains initially and then, as the easy fruit was plucked, the changes would give less and less and the successful tests would become fewer and fewer?
Likely yes. But what matters is return on time in contrast to any of the other things you could be doing. "always work on the most important thing". Even small percentage point improvements in conversion go directly to the bottom line, which is far from obviously the case when fixing bugs or adding features. And if your bottom line is already pretty big, the marginal value of someone working on optimization is likely to continue to be high, regardless of how plucked the fruit seems to be.
