Remixing off the shelf isn't really the issue if you want to compete, it's the fact that you wont even be able to buy the competitive components at all because they wont sell you them even if you ask and have the money.
That’s covered by the compulsory license part of the comment. Say Apple has a patent on the iPhone, Apple must license the iPhone itself to me at a reasonable fee. Then I can go sell jPhones all day to compete.
I don't think you could compete with the exact same product... the advantage of scale would still give Apple more than sufficient edge.
However if you happened to like E.G. Apple's screen, or camera, or another component that was better than what else could be selected it could be part of a design which competed on other merits. E.G. maybe the touch digitizer is just that much better, so it might make sense on some models of Android or some Libre phone more closely based on Linux or BSD. Or some company that makes an iPhone like device but to GOV spec standards for a given country. (In my mind, I'm thinking US Gov, but IP laws tend to be International too, so maybe Germany wants it's own secure phone.)
Who are you demanding from if there's no incentive to invest? Either the option is there, in which case the incentive clearly must have been there for someone, or it's not, in which case the incentive is the same for wanting to have it in the first place.
Hmm. Depends what you're referring to - iPhone is indeed a closed system, by contractual arrangement, but Android isn't, and it absolutely is feasible for third party Android devices to exist. Occasionally someone does a new phone startup.
Not especially a matter of patent, just good old fashioned contract exclusivity.