Why would a society who imagines a great future to come, imagine that their music and cultural tastes would remain stagnant. When they imagined a future car, they did not expect it to be a 57' Chevy, they expected it to be a design of the future ~with 50's aesthetics. The same goes for housing, and architecture. Look at Chicago in FO:Tactics; it's not 50's high rises and bungalows, it's silver spires. It's closer to The Jetsons.
So why would their future descendants want the musical equivalent of a 57' Chevy (songs they've replayed for decades) instead of futuristic new music ~with a 50's aesthetic.
The (in world) music that Fallout ~should~ have had should not be pre-60's music; which was used in the game intros as a remembrance of the world as it existed deep in the past.
The music of 2075 should have been from new artists with their own ideas; not archaic cover songs from the 1950's, not 150 year old relic tunes from the past. Their music would still be in a noticeably 50's style, but noticeably NOT from the actual 1950's.
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