Namor must have lied to those bad guys.
Because all you have to do to save the universe, is destroy the Earth.
No Earth = No incursion.
Meanwhile, on an omniversal scale they need to do is destroy all Earths, and maybe even destroy all Earths before any incursion cross their paths as a preventative measure, like how some women have double mastectomies to side step a genetic predilection for breast cancer... But, y'know, with planets.
No Earths omniversally = No incursions omniversally = Omniverse saved.
The only way Namor could talk them into saving the multiverse by destroying an infinite number of Earths which are threatening Infinity x Infinity Universes is if in that bargain he stipulated that his Earth, which no one in the Cabal gives a damn about, is destroyed last.
Which is a shit deal, considering the risk to the Cabal.
If the 616 had no Earth to be a shit magnet for incursions, the Cabal would have to go out hunting the scavenging for Earths (no matter what, they still have to do this.) and Incursions to save the day which would be exhausting.
By leaving their own native Earth in play, it draws in the Earths of other Universes which Thanos (et all) can then destroy the earth rear-ending 616's earth and therefore save the universe this bushwhacker is sitting in from being destroyed alongside/along-with it's Earth.
I have no idea why Doctor Strange has not talked to Eternity about this ailment he has?
Maybe he just needs some aspirin?