Had this pegged at $80-$90M for the 4 day, which was reasonable and perfectly fine. Never was this going to topple (let alone match) Days of Future Past's 4 day, or domestic gross.. or worldwide gross and in their case two years ago it also fell below expectations stateside.
I love them but I'll say it. The First Class cast was never hot with the masses (Remember how so many swore it would break out to $200M+ box-office back in 2011? I do.. and I specifically remember saying it would be the lowest-grossing installment). So much so their own sequel (not unlike what happened to the Man of Steel sequel) was morphed into Days of Future Past to integrate the "old favorites" back instead. Gave the example a few days ago that going back to the First Class cast would be like returning to the Tokyo Drift cast after Fast & Furious 4.
Thing is keep in mind is this. The Wolverine remains the lowest grossing installment in the franchise domestically. At yet Fox was beyond happy with its overseas performance and OKed a third film. So anyone proclaiming "THAT'S IT! IT'S OVER! REBOOT TIME!" needs to sit in the corner for awhile.
They've been thinking big picture ever since then, and if this does $500-$600M worldwide (as I expected), then you're still "stuck" with this.