I love the whole "lost Saturn V engineering" trope. It sounds so good despite being bullshit. Rocket dune and NASA did an extensive knowledge retention project for the F-1 and J-2 engines. NASA also maintains several copies of extant F-1 and F-1A engines. If we approached Rocketdyne tomorrow with a billion dollars they could restart F-1A production. The J-2X is in fact the second stage engine for the SLS stack.

The SSME was selected for the SLS (and Ares V before it) because it had twice the burn hours of the F-1 and the SRBs existed and were a known quantity. The SLS (and Ares V) can reuse more of the Shuttle's infrastructure than an F-1 based design. The SRBs allow for use of smaller first stage tankage and the less powerful SSMEs which allow for reusing Shuttle launchpads which replaced the Saturn launchpads at LC-39.

While the SLS is moving into boondoggle territory the Ares V and SLS designs made economic sense to reuse Shuttle designs and infrastructure. It didn't have anything to do with the myth of lost engineering knowledge.