Gonna jump in here "The end of the Sons of Sparda" idea was floated around by several journalist sites but never discounted by Capcom, while the Capcom Unity page states
"the stage is now set for the latest chapter of the “Sons of Sparda” saga" which while very vague on whether this is Dantes last main role or not, still sets this up as like Vergil has been an intergral player in the series which he has not been. (He may have been retroactively added into the overall series but it was an after thought)
The fight between Vergil and Dante in 5 is probably the most luke warm iteration in the series, the build-up is half cocked and the resolution doesn't feel earned. Whats stopping Vergil from doing something like this again? Certainly not Dante. And I would argue Dante giving Nero the DMC brand is more a torch passing moment (and more heart warming) than Dante just peacing out with Vergil into the Underworld, especially since they can leave at any time but the game acts like this whole thing is a big deal lol.
It was very clear to me that the ending of the sons of sparda simply mean the ending of dante and vergil rivalry...
The saga was originally going to end in dmc 1 since it was pretty conclusive, Capcom did not have a plan in mind so they simply add more content while making game but I think that dante and vergil rivalry and story was always present, we have it in dmc 1, it is the main theme in dmc 1 novel, in dmc 3, it continues in dmc 4 and than in dmc 5, a part of dmc story was always about dante and vergil...
I agree that the third part of the game should have been longer and should have been better to let us understand more about vergil heart changing ecc...
Whats stopping vergil from doing something like this again? Well certainly dante himself since he followed vergil in the underworld for this reason, also I don't think vergil will want to do something like this again since as V he understands the atrocity he made and how bad the crime he committed were
Probably you didn't understand that dante and vergil sealed the door to the demon world and Nero say that if they sealed it the can't come back...
They destroyed the qliphot tree than they sealed the demon world for good, re-watch the cutscene
They say this:
Nero: wait where are you going?
Vergil: we need to separate the qliphot root from the underworld itself, then we ll seal the portal whit the Yamato...
Nero: wait if you do that you can't come back
In my opinion This dialogue let you understand that if they close the demon world portal they can't simply open another portal to escape because it look like they seveared the underworld from the human world for good, it's like you entiring into a room (demon world) using a key (Yamato) but after you entered it you closed the door (the gate to demon world) whit the key ( Yamato) and than you destroy the door lock (demon world door) whit the key (yamato), this mean that you ares stack in the room for iternity... Or simply just ask capcom itself why they can't come back but for me they were clear enough