Of course TOS was good. It lasted three seasons on a REAL network, NBC, not a joke network like UPN, or on early-evening syndication safely away from primetime competition. Considering that science-fiction, especially space-based science-fiction, notoriously doesn't do well on the Big Three (now Big Four) Networks, then or now, lasting three seasons in that type of situation is pretty respectable. Then it lasted in syndicated repeats for almost two decades before TNG came into the picture, even though it had less than 100 episodes, people enjoyed watching the episodes over and over again, and it left them wanting more. That doesn't happen if the show isn't good.
And that's what Star Trek was: a TV show. Then it became the '60s TV Series that was turned into an '80s Film Series. It did what most shows don't do: it successfully transitioned from the Small Screen to the Big Screen and switched mediums.
Then what we think of as The Franchise started. TNG begat DS9 and VOY. All three ran seven seasons each. It didn't leave people wanting more. It ran and ran and ran until people were sick of it. ENT was dead-on-arrival because a lot more people were sick of The Franchise than not, and tuned out, including me. Now it's back, with the new shows, but behind a paywall safe haven. That's TV. Then there are movies. TNG didn't translate as well to the Big Screen as TOS did, and really finished its "film" run on Streaming TV after the last actual TNG Film flopped. Two decades after the fact.
So, clearly TOS had something the other shows didn't. Clearly it was easier to adapt TOS to movies than TNG.
BUT
The topic is about TOS's staying power with young people. That traditionally means "Under 30". To them, TOS is the first of a bunch of old shows and new shows. Maybe they'll like it, maybe they won't, it depends on their tastes. At the end of the day, if I want to know what people under 30 think, I'll ask people under 30.
Speaking as a 40-something, I'm not going to use what someone under 30 thinks as a barometer for whether not I should think it's good, because I don't expect their tastes to match mine. But if the generation
after me thinks something is good, I think it's good, and the generation
before me thinks it's good... then I think it speaks to the appeal of something across multiple generations. And that's huge.
Unfortunately, at this point, it's only going to be people who are open to older shows and movies who'll give it a chance. To put it into perspective: a 20-year-old today looking at something from the 1960s would be similar to me looking at something from the 1940s when I was 20. As old as TOS might seem to us, I think it would seem a great deal older to them.
I like movies from the 1920s, '30s, and '40s. I even recently colorized and upscaled an early talkie in the public domain...
... but I know being interested in films from that far back is a niche interest for someone on the Generation X, Millennial border. It would be a similar situation with a Zoomer watching something from the 1960s.