Nothing will kill the franchise. The worst that'll happen if Covenant bombs is it will go on ice again. Alien Resurrection and particularly Aliens vs Predator Requiem performed atrociously at the domestic box office, but here we are, a month away from (likely) the biggest budgeted Alien movie yet by a director who can choose whichever project he likes.
I think Covenant will break $300 million internationally based on figures from the previous instalments. This is my personal projection regardless of what word of mouth is like.
What worries me is the US reception and ultimately revenue. I would expect it to break $100million, anything less will be perceived as a disappointment. Obviously fantastic reviews will determine if it has legs at the US box office or not.
As a guesstimate, I say between $110-130million is the US (160 if the reviews are phenomenal). $400million+ total worldwide. I don't think it'll be close to $500mill though.