Every player will have CPU limit on how much time his script can use in milliseconds. So that's the challenge to make the AI strong and not exceed the resources.
Having one person being able to pay for more cycles is the textbook definition of unfair : it restricts the non-paying player to a subset of optimized searches, while the paying player can ultimately get an edge because of his additional compute.
Also, people are assuming a black market of scripts will develop just as it does for gear, etc. gear is different because the frequency table is fixed -- therefore the competitive landscape is fixed. When everyone can have 'nukes' the winning strategy is no longer 'nukes', it's who can sneak through defenses and disable the nukes, or confuse them.
Kauffman calls this constant evolution into the 'adjacent possible' -- and if Screeps even half delivers that kind of depth, there won't be a stable end state to the game where everyone has one master script. Changing environments necessitate changing response. You are part of the environment. Feedback and permutation!! Yes!!!