My assumption is that should the Enclave have successfully used their virus, they would have wiped out all of humanity exposed to radiation. Assuming that the Enclave wears its Power Armor to prevent exactly sort of exposure, this would mean that the entire sum of humanity would be only those people sealed in Vaults like Vault-101 and the Enclave itself.
The virus in Fallout 2 doesn't target anything other than humanoid creatures, a vaccine is required to survive it's effects.
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Assuming nothing went wrong (they HAVE been preparing for seventy years), this would result in the following:
That is an error on my part, they have been planning it for eight years; though they learned of the Mariposa Military Base 70 years ago.
2. Rebuilding the Earth would take far far far longer. The problem with reconstruction is creating new infrastructure. NCR has only managed to rebuild California in a century and it has a far larger population than the Enclave.
I'd say rebuilding would take, at least, ten thousand years or so. On average.
An obvious problem would, of course, be a very low population; but this only increases time and ties in with some of your positives.
3. Vast swaths of human knowledge would be wiped out and any information not stored by the Enclave would be lost.
I don't understand the differene between this and eight, almost all of the post-war world's knowledge has came from the pre-war world.
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Positive: The Enclave may or may not have Mister House's support (more likely not since they've murdered the entire population of New Vegas). At the very least, he'd be protected in cryostasis and be capable of interacting with the Enclave.
I imagined that they would not; House's Vegas would never be built and he would have very little to offer the Enclave, in-fact he would really be able to do anything.
10. The Enclave would probably make an attempt to get in touch with all surviving Vaults. This would, at the time of Fallout 2, include a few vaults in the Mojave and Vault-101.
So humanity would actually maybe have a small chance of survival if they could get their cooperation. Unfortunately; it would introduce insane gun-toting Rednecks (The Boomers Vault), Compulsive Gamblers, Crazed Botanists, and Isolationist Weirdos into the Enclave's gene pool.
This, potentially would mean the Enclave would have to go to war AGAIN with survivors (their massacre of the Vault family in Fallout 2's opening inclines me to believe the Enclave would massacre the Vault survivors as opposed to protect them).
I would imagine that they would protect them, I've just played Fallout 2 earlier to-day; when talking to Richardson and wearing a Vault 13 Jumpsuit he says:
"Ah Vault 13; your are performing an invaluable sacrifice for your country."
The massacre at the begining could simply be put down to Enclave soldiers doing what they do, Richardson treats you with distain when he knows your a tribal but as a hero when he thought you were from Vault 13. I think however that yes, those brought from the unopened Vaults could severly interfer with the typical culture of the Enclave; if not cause a conflict.