Then all they have to do is find better ways to invest it in growth or save it for when there is an opportunity. Of course they aren't saving for retirement, but they should be saving for something. A buyback is meant to boost investor confidence, but how much does that benefit Apple (especially if they don't expand the company any concrete way)? There could be a better opportunity to spend in the future.
I agree, they don't need that big of a cash hoard saved up. Apple needs to expand. But if a buyback is what they really should do, it should not be one the size of their entire cash sum.
The thing is Apple is investing. They are just doing it strategically and at a moderate pace. That's smart and better for the business.
Furthermore, Apple now has damn near infinite money for R&D and supply chain control and management, which again is more valuable to the business than Icahn. Thankfully Tim Cook does have the money and clout to rebuff Icahn.