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.
> Ares V and SLS designs made economic sense to reuse Shuttle designs and infrastructure
Theoretically made economic sense.
What would have been a more conservative design? Reusing the Apollo design was considered and discarded because so much knowledge and tooling had been lost that it was considered to be less conservative than the SLS design.