To be fair, ST5 was very explicit about the adventure only taking place a few weeks after the end credits of ST4. in ST4, Kirk said "Let's see what she's got", and in ST5 Scotty comments outright that it turned out to be the starship equivalent of measles...
But it's very true that after the three-movie arc of big things happening to our heroes, we now get treated to another standalone TOS adventure of the very classic sort. Which was a welcome change of pace, but then again, ST4 had been a change of pace as well... It really was time for a "scary" sixth installment to the series, but the actors could probably have gone on with yet more variations on the theme, had the pace of filming been a bit higher.
A second series might have been nice. But a series of movies offered bigger budgets, and forced the writers to do more per "episode", to be bolder overall. With just a bit more money, they could even have started tinkering a bit more with the hero hardware, perhaps giving Kirk a truly new ship, perhaps just showing us much more of her inner workings.