Plus, the Earth-Romulan War didn't start until 2156, which would have been during Enterprise's 5th season, if there had been one. So if they had spent a lot of time building up the war...it would have had no payoff whatsoever, since the show was cancelled before then. Further, the Earth-Romulan War lasted 4 years...which means Enterprise's 6th, 7th, and 8th seasons would have had to be about it, as well. I imagine if the show had run on that long, people would have been pretty tired of it by that point. I mean, just imagine that--a four year long arc fighting an enemy that the crew never even sees. :/