V's Deal: The Full Consequences

[Snails]

I think the reason was that Vaarsuvius is likely to be near one of the gates again sometime quite soon, where a random commoner probably won't ever be near one.

The (almost) new suggestion in the OP is "...the fiends may be able to bring their immense power into the material plane!" If that is really the game, then why not just seduce a commoner and seize the Gate, steamrolling over Xykon like a speedbump?

Now, it is possible that the OP is actually correct -- we cannot disprove it. But we do not have data pointing in that particular direction, and it actually creates many more difficult quandaries than it solves.