They easily could have made the 5th season all about the Terran-Romulan war but tempered it with many stories within the big arc, the same way they did with the Xindi. I actually thought the 3rd season was where the series got good. From a fanboy perspective, the TOS homage stories were great and I wouldn't have minded more of them- as long as they were written well.
"Stop it? I'm counting on it."
"But not because you threaten me. I'll pay you because... it's my pleasure."