Is what Trek has always done. TOS is American culture of the late 60s. TNG is definitely a period piece as you say, it is a period piece of late 80s America. They may portray people in the future but the things they care about, the attitudes they have, the way they approach problems and the stories they take part in all reflect strongly the times they are made in. The latter Berman era shows were trying to replicate TNG so are less reflective of their own periods, but the comparison is back with JJTrek and the late 00s. Undoubtedly, Discovery will be, perhaps with the benefit of hindsight in years to come, a mirror of late 10s American culture.