They clearly didn't have ANY backup servers to commission in case of heavy load. That is just atrocious preparation and lack of foresight by Square. P4 should've at least clued them in that there were more NA/EU players than they originally expected.
And yet, not a single new server has been opened despite nearly all NA/EU servers being locked for character creation
Sure, they don't want to go overboard like SWTOR did, but I suspect they didn't have any extra servers ready to go for a situation like this. And it being a weekend meant nothing would be able to be brought in until Monday. Their "fixes" during maintenance reflect this. Instead of increasing capacity to handle the load, they're capping the number of players who can log into the game itself. So now instead of putting strain on the instance/duty servers, we're straining the login/lobby servers. Good job Square.