But that’s the thing - they aren’t. Not once they’re bought. At that point, they’re my device, or your device.

Well, you may want it to be completely yours, but in fact, there are many things that you can't and sometimes don't want to control on your phone. But Apple never claimed that you can control everything. Apple never advertised their phones as having many application stores; quite the opposite, actually. You don't expect a satellite connection from a phone that doesn't have it; you don't expect a phone without water resistance to work underwater. I understand if some product does not meet your expectations, you're frustrated, but in this case, you received exactly what you asked for. Want something else? Buy from another company. Why force this company to do things your way?

Surely you can see how having a single supplier can be a bad thing, right? That supplier has no incentive to deliver quality. Why would they?

Of course, I can see that having a single supplier can and will cause many issues. The problem for me is that I don't believe in monopolies. Monopolies are very unstable. Firstly, for a monopoly to form, a few things with low probability should happen: in your analogy, there should be no other cookie provider (neither now nor in the foreseeable future), and customers should be willing to buy cookies that I produce at any cost. In reality, there's always someone else who's willing to (or at least can) produce more cookies, and customers are not complete idiots. If I increase the price or lower the quality beyond their limit, very quickly I will be left with full warehouses and a bad reputation and go bankrupt. Secondly, you always have a choice. Present me with a situation, and I will tell you which choices you have (they all may be bad, but whatever they are, they are options). In the case of Apple, there are obviously plenty of choices. They're not the only company producing smartphones. And even on their phones, there's Cydia. So, what monopoly does Apple have? Well, they're the only corporation that can produce iPhones. Should we allow other companies to produce iPhones in this case?