Sure, we can assume that, but is this assumption based on them optimizing their CapEx with regards to their current product mix or with regards to future expansion? Apple is a very large company that's reliant on very, very few revenue streams and very very few product launches to keep itself afloat. To me, optimal capital expenditures would allow for being able to smooth that out with a somewhat more varied product line, with launches throughout the year. Right now, Apple is like a pharma company reliant on only one drug, and nobody knows if there's another blockbuster drug coming down the pipe anytime soon. But I'm digressing.
Ok, optimal CapEx, impressive vertical integration with no production shortages (which isn't the case), growing product portfolio (which isn't really the case), stable quarter to quarter income (which isn't the case), all assumed in this scenario. I would then use this low-risk opportunity to look at the pricing sensitivity on the highest margin products - in this case, iPhone and full size iPad. Given that everything's optimal and there are no product shortages, a reduction in margin on each device sold should only increase the number of devices sold, which should allow for an increase in market share and a more market-dominant position where you can afford to use your second mover advantage. Arguable whether or not Apple's in this situation right now given the high-teens iPhone marketshare.
The flipside of the argument is "what if price sensitivity is inelastic and there are no more incremental sales from lower prices?" Given that the market isn't saturated, that then means that someone else is fulfilling a need that your product can't fill, which then goes back to the question as to whether or not your optimally spending the money that you have.
Now, if we assume marketshare dominance (not just mindshare, or even just cashflow dominance), then yes, Apple could start buying back stock as it would then assume the position of a utility, one where incremental CapEx would merely be viewed as a cost rather than an investment.