I think the key point to take away from the OP is within Trek fandom. It's only natural as time passes and more and more newer Trek productions arrive, that the older Trek shows wane in popularity among younger/newer fans. For most, TOS isn't their Star Trek. They didn't grow up or start with it and never developed a personal attachment to it. For some, TOS is totally unwatchable because of its production values, sensibilities, and aesthetics aren't compatible with what they are accustomed to or simply prefer. Just because TOS was the first Star Trek series and is what all others have derived from doesn't mean they're obligated to treat it like it's holy. As said earlier, it isn't their Star Trek series, and that sentiment can only grow more widespread throughout Trek's fandom in the future--either due to more newer fans who favor the newer productions or less older fans who still love TOS (or at least won't throw salt on it. There's truly nothing wrong with that.
But outside of Trek fandom? In overall pop culture, TOS will forever retain its iconic status alongside I Love Lucy, Elvis, the Beatles, Gone With the Wind, etc., that have all withstood the test of time even if they are products of bygone eras. They are historic and will always remain legends.