Based on that issue, nobody is still planning to save the universe. Their goals is to allow some people to survive the final incursion, one way or another. In that respect, Scott's plan is not more doomed than anyone else's. The White Phoenix (and even more the cosmic Phoenix) was shown, several time, to be able to "rewrite" reality, destroy/construct whole worlds (see NmH for example) and, as important, was shown a willingness to do it for Earth and humanity.
The various cosmic entities that died weren't trying to save Earth but to fight the Ivory Knight. Scott plans is not to fight them. But to survive. And Phoenix would certainly help him doing so.
Some people are looking for a technological method. Others at a "super powered" one. I am just suprised that they didn't showed us people planning for a magical method.
I doubt that its about building an ark or something like that. Not with the power of the Phoenix. And there is no reason that the Phoenix would limit itself to mutants if it can save Earth or humanity from the final incursion.
The technological ark stuff is cute but how many people may they expect to save that way? A few hundred? Maybe a few thousand if we are very generous. But that's about it. Even if the whole Shi'ar fleet was helping evacuate the Earth, they would only be able to get a very small part of the total population.
In NmH, when Magneto asked, the Phoenix said it would consider creating a world in the void between the universe (of whatever it is) for the mutants from the various alternate universe that are persecuted. Scott is probably hoping for something like that. Or at least having the Phoenix help part of Earth to transition to post-final incursion. (And there is no reason that Scott and Phoenix would not do it for non-mutants too if they can)