Someone didn't have to go, they've been focused on the 2017 car for 2 years, you can't make dramatic comebacks against a team as strong as Mercedes that has made a car as ridiculously strong as it is. But new car, new regulation period, that is when the big changes mostly happen in terms of running order.
I suspect the real story is Allison needs to be at home, he's not been to races since his wife's death and he's apparently been travelling a lot between his home and Ferrari's base on Italy which is ideal for no one.
Most likely Allison decided to leave and Ferrari being a publicly traded company and not wanting to say publicly that they are losing a crucial member of the team which looks nothing but bad, asked Allison to play along with a story that changes were needed for the good of the team and probably gave him a bigger pay off for going with that story.