Ask Pell, Conis' father back in Skypeia, all the fodders in Thriller Bark, and all the fodders in Punk Hazard.
He just felt like those arcs deserved a good ending for the good guys. Monet was stabbed right in the heart while Vergo got caught in a huge explosion. I don't see either having survived with the possible exception of Vergo but I doubt it and there is no reason for him to survive. Law got his payback, Doflamingo himself is down, why would Vergo still be around not to mention we now know he's fodder compared to Luffy and Law.
