I would have liked more N'Zoth and less Sylvanas as well, but oh well. I personally doubt that even IF we kill N'zoth the Old Gods will not reappear. As someone already said Xala'tath is out there and already promised a reunion and there is Alleria too who might be having a directly line to the Void Lords.
Besides in the grand plan of things the Old Gods were only ever flunkies of the Void Lords. They are only "gods" because they declare themselves to be, they are much like Il'gynoth simply sentient tumors sent to grow and overtake the soul of our planet. That is why their plans rarely involve our collective extinction (contrary to whatever Sylvanas is up to). We are not their enemies, they see us as servants willing or unwilling, but they feel no hatred towards us.
In fact I believe that N'zoth very much wants to stop Sylvanas and by defeating him, we prevent him from doing that. It would have been incredibly cool if we could make a pact with him at the end of BFA to fight Sylvanas and her Jailer BFF with the power of the Void. If indeed the Light has made a pact with Death (the enemy of all) then the Void making a pact with Life (represented by us) would have been a sweet deal.
If people get to be Sylvanas loyalists I feel we should be allowed to be N'zoth loyalists with our own story line