There was of course a ton of testing at the component level, and iteration when necessary. This worked for Saturn V because it could be done for each component

There was of course a ton of testing at the component level, and iteration when necessary. This worked for Saturn V because it could be done for each component in parallel, due to the huge mid-sixties spike in funding. But at the level of the integrated system, most of the testing—and nearly all of the iteration—was cut out, since it would have to be done in series for the entire rocket (or its stages). Thus the design had to be set in stone before the vehicle ever flew. The first and second stages of Saturn V flew just twice before crewed flight began. This was the right process for Apollo’s unique set of requirements and resources, but it is a bad idea when you A) don’t have a hard deadline forcing you to forego system-level iteration and B) don’t have a spike in funding to accelerate the up-front development, which otherwise takes forever. Hence SLS.