You still have yet to actually prove that Necron is omnipotent. Burden of proof is on you to do so.
Aside from him being able to wipe everything to perpetual nothingness (existence itself, not some mere universe)...
He is truly immortal so long as there is life and death which, in itself, defines existence. If he destroys everything and then creates everything again he will once again exist while at the same time being the only thing to exist even in nothingness... if you can even qualify zero point as being "nothing". Sorry, but Demonbane doesn't even wield a fraction of just that power alone... this is the difference between mortality and Necron.
His appearance and reason for being there at the end of the story, Kuja's connection, the world's connection, desire for nothingness of the world... Kuja... mankind.., etc. He was not defeated but simply decides to observe human's will and power... when he vanishes the eyes closing and his speech as well as continual existence. Life and death itself.
There is quite a bit more but I'm not really going to put up with all that effort.
Burden on proof is on you... What leads you to believe that Demonbane can destroy something truly omni-potent or erase existence, itself? He can't. He is effectively the equivalent of max stats but those stats are not infinity.
So off topic... and we are the only ones even discussing in this thread now.