In particular, and apropos to the comment you are responding to, we have a wonderful new way to weld aluminum alloys: friction stir welding. It was invented in the 1990s, long after Apollo.
Despite modern techniques being superior, I share the sentiment of the author that the lost knowledge is something to be mourned. Even though that process wouldn't be used in a rocket today, it is possible that technique could have inspired a breakthrough for some other new process. Like how Gorilla glass, originally developed in the 1960's, was seen as a dead-end and largely useless, until the iPhone was being designed.
We didn't lose the knowledge of making chemically hardened glass. It just didn't have a lot of uses until everyone had touch screen cell phones that needed to be scratch resistant.
Patents are supposed to document inventions well enough that they can be reproduced. But I wonder if the background knowledge can decay enough that this stops being true. And I wonder if enough decay occurs, could things that were previously unpatentable for obviousness now become patentable again?
"United Launch Alliance applies FSW to the Delta II, Delta IV, Atlas V, and the new Vulcan expendable launch vehicles along with their Cryogenic Upper Stages, and the first of these with a friction stir welded interstage module was launched in 1999. The process was also used for the Space Shuttle external tank, for Ares I until the project was canceled in 2012, The SLS Core which replaced the Ares, and for the Orion Crew Vehicle test article and the current model of the Orion at NASA, as well as Falcon 1 and Falcon 9 rockets at SpaceX.[53] The toe nails for ramp of Boeing C-17 Globemaster III cargo aircraft by Advanced Joining Technologies[7] and the cargo barrier beams for the Boeing 747 Large Cargo Freighter[7] were the first commercially produced aircraft parts. FAA-approved wings and fuselage panels of the Eclipse 500 aircraft were made at Eclipse Aviation, and this company delivered 259 friction stir welded business jets, before they were forced into Chapter 7 liquidation. Floor panels for Airbus A400M military aircraft are now made by Pfalz Flugzeugwerke and Embraer used FSW for the Legacy 450 and 500 Jets[8] Friction stir welding also is employed for fuselage panels on the Airbus A380.[54] BRÖTJE-Automation uses friction stir welding for gantry production machines developed for the aerospace sector, as well as other industrial applications.[55]"
