Why? Humans can't attack the idle gods. Can't attack Winter or Summer. Cannot attack Hades or Zeus. Humans are limited to fighting in the Earthly plane. Why would Mab, or Hades, or the old gods get involved against humanity in a war with say the Fomor? What would the benefit be? In actuality these gods would see it as an opportunity to remove the Fomor. There is no reason for the gods to side with vampires, fomor, ghouls, or any other Earthly realm beings. The gods are beyond humanities reach, and are no friends to most of the supernatural creatures who call Earth their home.
Because the proposed attack wasn't on the Fomor. It was in the Whamps. Lara would not sit idly by as her people were killed. She'd go after whoever was responsible. So it'd either end quickly, with the humans smacked down and put in their place, or it'd escalate.
If it escalated, with humanity bringing their weapons to bear, then the opportunistic sort like the Fomor would back the Wamps. The Sidhe Courts can't work together, so you'd have two of the most powerful factions ready to fight the humans to prevent their extinction, with the others limited in what they could do. And you know Cristos would be warning the wizards that the vanilla mortals would turn on them at the earliest opportunity, just like they did during the witch hunts.
And how many other minor gods out there are weak because of a lack of faith and opportunity? How many others would love a return to the old ways? To scare the hell out of the humans in their domain, feed off their compliance, and grow stronger?
The humans would fight back, and so would the supernatural. Some mortal working for the government would figure out a way to open a Way to somewhere important (Arctis Tor, perhaps, with the help of Cowl?) and they'd send a nuke in a la the World Council in the Avengers, or the US in Independence Day. And then the Courts *will* be involved, and not on humanity's side.
The supernatural creatures would just open Ways into every nuclear missile silo, storage facility, and sub, and just take the entire arsenal. Butterfly-winged faeries would carry them in orbit and above the cities and set them off, knocking out satellites and electronics. Buried phone and internet trunk lines would be broken. Power plants would be sabotaged even worse than they were in DB, sending the population into the dark ages.
And then the old gods would come, making humanity capitulate. Many would fight, but we're talking maybe 50 million military personnel to protect 7 billion? With no nukes, against an enemy that has them, that can go ahead and nuke every military base and port?