If the species who formed the Coalition of Planets never came together, or even became particularly close, I wonder if this alone would have prevented the Romulan War?
With a continuation of the pre-existing status quo, the Romulans would have been content to remain in the shadows. There would have been no growing political unity for them to fear and seek to disrupt.
The butterfly effect.
The Humans would have expanded outward from Earth, seeking out trade and colonies and discovery and adventure, a few nuggies from the Romulans and we would have simply avoided their space, venturing in every other direction.
There are interstellar threats other than the Romulans and Starfleet would have grown and adapted to handle them, but with out the events of the Romulan War, Starfleet would have remain a relatively small to medium sized military/exploration organization, suitable to the protection of Earth, Human trade routes, civilian shipping and colonies.
Earth would have been the Switzerland of the galaxy, friendly to most, but also very independent. Helpful perhaps in assisting others in their negotiations, but never taking sides.