^ Why?
It doesn't much matter if he is his son or not, it's not going to affect the story because "Vergil" is dead. Unless y'all are riled about what's even sillier than Nero being Vergil's son - the prospect of Vergil being resurrected (from DMC4 onward in the series' chronology). I mean, Vergil chooses his path, becomes an evil minion, and dies by Dante's hand, The End. While Nero could be the son of (corrupted) Vergil, 'bringing Vergil back' as he was in 3 could be some icky retcon mess, and bringing him back as some other form of himself might be okay, but it wouldn't be 'him' any more than Nelo Angelo is anything like DMC3 Vergil.
Is it because the idea compromises Vergil's character from DMC3? How? I mean, Sparda was apparently this badass with a outwardly cold persona, and he still managed to have kids. It's not an impossibility.
But even so, it just doesn't seem that important to get really bothered about. Now if they brought him back with a totally lame excuse to, I'd be bugged about that. But I empathise with fans who're annoyed at the randomness of Nero's introduction to the series and the 'hints' about him being Vergil's kid. That's annoying, because it's playing with our sense of recognition of the characters. But the idea of Vergil having a kid per se - I'm alright with that