I don't care about the cost-wise reason or version-wise reason: if 343 is going to make more Inifnity-class vessels, that'd be awful and unseemly. Just think about a bunch of over 5km long ships hanging around, or even worse, considering 343's attitude, a bunch of mini Infinity-class vessels...God why.
Every sci-fi universe has to have a symbol-ship. The Executor, the Battlestar Galactica, the Enterprise just to name a few. There must be only one of them in order to give it its importance.
Well...there are multiple Executor-class ships, Battlestars, and Galaxy-class ships. But I get your point. Infinity should be unique. She's kind of like the Destiny Ascension in Mass Effect.