It was claimed in Roddenberry's TMP novelization that Kirk was the only captain to return from a 5-year mission with both ship and crew mostly intact. We know that at least a couple of Connies, Excalibur and Exeter, lost their crews but physically survived. So it was never claimed that every other Connie was physically destroyed; Roddenberry's claim would've meant that some were destroyed while others returned/were recovered without most or all of their crews.
But I never found that claim likely or appealing. I don't like the idea that Kirk and his crew were so much better than everyone else. Sure, Kirk liked to say he had the best crew in the fleet, but what captain wouldn't say that? It doesn't mean the rest of Starfleet was so incompetent that over 90 percent of their capital ships failed to return from their tours of service. That's just inane.
I always took that as Roddenberry stressing just how dangerous deep-space exploration was and that Kirk and the gang were more lucky
than anything else. In the book, Roddenberry had Kirk privately deeply regret losing nearly 100 crewmembers during the course of his mission because he didn't always make the right decision at the right time.