The dominance of Microsoft actually depended on openness.
The IBM PC commoditized hardware and software, which facilitated PC clones. When IBM licensed DOS from MS, it allowed MS to license a version to non-IBM computers too. Thus, IBM ended up commoditizing itself and became replaceable, with the MS OS as the new standard.
Apple can lock down iPhones because it's the sole provider of the software and the hardware.
The IBM PC commoditized hardware and software, which facilitated PC clones. When IBM licensed DOS from MS, it allowed MS to license a version to non-IBM computers too. Thus, IBM ended up commoditizing itself and became replaceable, with the MS OS as the new standard.
Apple can lock down iPhones because it's the sole provider of the software and the hardware.