NASA engineer here. I think it's actually kind of ludicrous to claim that we have somehow "lost" the technology of the Apollo program. We're not living in some sort of space technology dark ages here, where we've forgotten all of the fundamentals of the 1960s 'classical enlightenment'. In fact, the decades since then have provided invaluable experience in the design and use of reusable launch systems, in-space assembly, astronaut medicine, etc.
If the agency and it's supporting industry contractors were given both an executive mandate and the funding to construct a comparable system to Apollo (for whatever reason that would be), today's engineers would not struggle to do it for lack of technical prowess. We might struggle to do it in the current federal budgetary climate (where we can't predict the whims of legislators 3 months from now, much less 10 years from now - and the Saturn V was expensive as hell to operate), but the "lost rocket science" myth is a bit tiring.