They're a public company. If the company isn't growing, there's little incentive to hold the stock. (The current 2% dividend is a pitiful return, you can get that level of return from an Australian savings account.) If they're not growing & the dividend isn't big enough, then the stock price (and hence company value) will start falling as people cash out.
If Apple can take themselves private again, there will be less pressure on them to keep growing.
If as you say the stock price falls, Apple could just buy back their own stock. They have a huge cash reserve.
What I'm getting at is I think everybody is overly concerned that Apple isn't releasing any "new" devices. They don't really have to. Looking at market share, they completely dominate on the products they already have. If anything, they can sit back, wait for the "next big thing" and do what Apple typically does best: Copy it, and make it so much better.
If Apple can take themselves private again, there will be less pressure on them to keep growing.