Because nothing lasts forever. Take a look at what Harmony OS is capable of if you want to see what a modern take on an OS and ecoystem looks like. It sure isn't the pinnacle either.
Will things change? Perhaps if/when quantum computing becomes a bigger item.
HarmonyOS is interesting because it points at the right axis: one coherent OS fabric across many devices, not a set of separate device OSes glued together by continuity features. Continuity, iCloud, Handoff, AirDrop, HomeKit etc are impressive glue. They are not the same as one logical OS fabric.