Every. Single. Episode. And. Movie.
Nice to know people living longer is less plausible to some than hyperwarp reptiles.
Those are different. Remember, we don't see every single day of the characters' lives. We skip over the majority of days where routine things happen, and focus on the moments when extraordinary events happen. So it's not a representative sample of the entire set of days or events. For every mission interesting enough to get an episode or book about it, there are probably a fair number of missions that are too ordinary or uneventful to be worth showing.
But if we're talking about the entire command crew of a single starship, then we're not just looking at the exceptions, we're looking at the whole thing. If you say that one, even two of the people in that command crew lived improbably long, yeah, that
could happen. But if you're talking about the entire command crew, then it follows that not
all of them would live improbably long. The more TOS main characters you want to live into the TNG era, the more ENT main characters you want to live into the TOS era, the more progressively ridiculous it gets.
It's also intellectually and emotionally dishonest and an insult to the intelligence of the reader. Death happens. We all have to face the loss of people we care about. So fiction shouldn't cop out of having characters face that loss. Good fiction is not about going easy on characters. Yes, improbable things happen, but they happen because they challenge the characters and raise the emotional stakes for the readers. If they don't do that, if they coddle the characters and the readers or make things too easy or comfortable, then that's generally a reason to avoid them.