TOS, like TNG, DS9, and every other show ever made is, simply, aging
. None of them were perfect when they were on air. Why anyone expects them to be anything other than imperfect now, particularly after watching various Trek
episodes and movies for decades
, makes no sense to me. They are all products of their time, and yet they all have episodes that continue to resonate for audiences today.
As to the specific criticism of TOS pacing, there's one tangible reason why modern audiences may pick up on it: Because there was less commercial time during the airing of TOS, the individual episodes are longer. Five minutes or so may not seem like much, but modern audiences are conditioned for shorter run times.