While I'll admit that a lot of the technology from all the series is beginning to look dated, the fashion/architecture/design of TOS seems more futuristic to me than the more "contemporary" look of the later series. This is probably because I lived through the 80s and 90s and did not live through the 60s.
The thing is though, we simply DON'T KNOW what the future looks like, so we're in no position to judge. Most things that go out of style eventually come back or at least inspire new styles. A lot of modern Apple products seem to take design inspiration from 1960s Braun electronics. Janice Rand's beehive looks more futuristic than Ishara Yar's mullet, because 60s hairstyles came full circle and inspired mid 2000s hairstyles. The mullet has yet to do that (here's hoping it doesn't).