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.
Can you give a brief comparison? I'm not familiar with Harmony OS and wouldn't know where to start on comparing the two.
There's no reason for Apple to migrate; none at all. The idea that they need to do so is just ridiculous, regardless of what Harmony OS may do. iOS/macOS does exactly what they need.
Will things change? Perhaps if/when quantum computing becomes a bigger item.