I already have explained; there's no alternative to iOS for apple mobile devices unless you're willing to pay more than $500 for an equivalent level of android device. If Apple allows Android to be installed to Apple devices, then things can be different though.
In fact software monopolies on hardware isn't pretty much the standard, it's a universal reality in just about all consumer products except one—the personal computer. And even then it's exceedingly rare for a consumer to deviate from the shipped software.