zOS is still the dominant OS for mainframes (60 years later!), MS-DOS was replaced by a product from the same company, Windows, that is still the dominant OS for desktops (30 years later). I don't see anything on the horizon that threatens zOS, nor Windows. Except their platforms disappearing, which is still at least 10-20 years from happening.
iOS is the dominant OS for its platform and Apple won't abandon it until the platform dies (I guess once we have wearables that completely replace smartphones?), Android is the dominant OS for all other smartphones. Microsoft poured billions into creating a smartphone app ecosystem and failed. Blackberry, Palm, Nokia, Amazon tried and failed. How, pray tell, would Android die if smartphones are still popular? The Google moat is HUGE.
Once you have critical mass, failure is next to impossible, slow and even preventable. Microsoft has been dominating desktop computing since 1981 through MS DOS and 1991 through Windows (almost 40 years!). Android and iOS will probably be around for 50 years, even if they become just another layer. We still have radio, TV, heck, even the post and vinyl.
Computing has changed. It crossed the chasm.