This narrative (presented in this image) doesn't seem to have a decent conclusion. And it's by a writer?

I think I understand what he's saying: He feels that Twilight becoming an alicorn princess would have been the way to conclude the series (i.e. "The End"). Perhaps he feels that continuing the series after such a momentous event would be superfluous… I think it is perhaps a valid criticism: indeed, a series running out of things to say is essentially the definition of "jumping the shark"…

However I also think that we should wait until the waterskis hit the water before we decide that a jump-the-shark moment has indeed taken place.