I agree that its conclusion was definitive, and you're right, for centuries, it's been contested as to Spartacus's (and Crixus's) intentions after the main army split--so the show's writers would literally be declaring what's historically accurate (a critical failing of previous dramatizations of the 3rd Servile War).
That said though, while Gods of the Arena through Vengeance had, in my opinion a very natural balance between content and story pace...War of the Damned seemed a little rushed to me.
You have 2 seasons that cover 1 year (73 BC) ...and 1 season that covers 2 years (72-71 BC). and you really feel it too...with very brief transitions between key events and massive character changes occurring frequently.
Let's be honest, Crixus's army barely got any camera time...and that was a major plot shift. Also, in terms of story development, the last season emphasized the development of Spartacus and Crassus and the similarities of their respective personas...but the culmination of which, in the final battle, seemed anticlimactic. Likewise, with as much time as they spent substantiating Crassus and Caesar, and as little relative time they spent introducing Pompey, that element also begged for a little more fleshing out (perhaps a mini-arc (a la Gods of the Arena)...particularly when those 3 characters, and their subsequent seizure of the Republic, were arguably just as (if not moreso) influential and historically remarkable as Spartacus's legend itself.
Ultimately, if I were to do one thing to change that series, I would have added an extra season's worth of episodes: half covering the skirmishes after their escape from Glaber's troops at Mt. Vesuvius...half covering the final battle and the aftermath.
(Note: for those that haven't seen this show, I'd highly recommend it...it's from the makers of Hercules/Xena (Tapert/Raimi) ...and it's like they took that early, casual glimpse of Ancient Greco-Roman culture and injected it with a gallon of steroids, giving it a real grittiness. After a few weekends of binge-watching, I was racing online to learn more about the real historical events.)