Why do you assume that an attack would take 1 year, and not (e.g.) a billion years? A factor of two is only interesting if the number you're dividing was interesting in the first place.
imagine is hardly assuming. But it doesn't have to be exactly 1 year, any attack that takes longer but less than 2x average certificate lifetime with 64bit serial numbers (useless) becomes practical on 63bit serial numbers (useful, for a strange meaning of useful).
> Any such attack would also become feasible with twice the budget.
Assuming that the attack yields to parallel computing and scales linearly with more cpu/cores, because linear programming is bound to current compute capabilities and then theoretical limits like Bremermann's limit and Margolus–Levitin theorem.