Zidane: Yes, Necron's appearnce was sudden but admit this, it was sudden what kind of form of Zeromus you had to fight the end. It was sudden you fought Neo ExDeath, at least it was for me, because I was expecting the final boss to be the guy who originally tried to take the Void, Envo. In Chrono Trigger, when you fought Lavos it was sudden how many forms you had to fight. RPGs are made on sudden plot twists, if you don't expect a sudden plot twist at the end it makes for a boring game, for example in FFX I found Yu Yevon easy, I said "please let there be a second form, there wasn't, and that made me mad with FFX for a long time.
Appearing suddenly and appearing suddenly with no/little plot direction are two different things. At least Lavos was in the story previous to the end, same with X-Death; Necron just...showed up. The little evidence I can remember to him even being there is when Garland says he's waiting for a time when there's no life or death, and even that's a stretch to use for Necron's entrance.