True Left

"is it true that Counterculture-influenced post-Counterculture musicians may surpass non-Counterculture-influenced musicians of the same time"

Success is often unrelated to quality, so if by "surpass" you mean in terms of success, then I would not anticipate a correlation. If you mean in terms of quality, then of course I myself am partial to post-Counterculture works that pay nostalgic tribute to the Counterculture era.

"but gradually loses the quality of a melody (as well as lyrics, talent, and similar traits of a song/musician) to so called “quality” of production (as well as trends, marketing, and similar advantages unrelated to the song/musician)?"

If the intent is sincerely tribute, then the production can be expected to deliberately avoid overly contemporary elements. Whereas if the production incorporates these while claiming to be tribute, I would doubt the sincerity of the claim.

A movie that I consider the best post-Counterculture-era movie known to me is an overt Counterculture-era tribute:

https://yts.mx/movies/dance-with-me-2019

Not only are almost all the songs from the Counterculture era, but for marketing they relied on a retro dance tour with local participation:





What do you think? (Also Aryan phenotype alert.)