A little crossover is fine between the two OSes, but to think that there is one OS that will serve both the desktop and tablet market is a mistake. Bringing key features from one into the other is fine and creating a common usage metaphor is alright but the tasks are different, which means that core functionality is different. Now, you could theoretically creating an adaptive OS that will change with the environment (e.g., touch vs mouse-driven). That could be potentially good on a touch enabled laptop or a tablet with a keyboard (e.g., Surface). There are challenges to that and the potential for failure would be high.
As far as abandoning Intel for ARM on the Mac, that would be a huge mistake IMO. Yes, Apple has been able to successfully make that transition twice (Motorola 68K -> PowerPC -> Intel), but ARM is not a desktop processor and the Intel architecture still has a lot of life in it. What professional Mac users often require is compute power for video, image editing, modelling, etc. Unless Apple is going to abandon the high-end market and just focus on consumer-level Macs, moving from Intel would be a huge mistake there.
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