The instruction set matters less for performance than people think. But it does matter a lot for the ecosystem: low-level hyper-optimized libraries for certain

The instruction set matters less for performance than people think. But it does matter a lot for the ecosystem: low-level hyper-optimized libraries for certain operations (think BLAS), tooling, developers understanding how to best write for it, etc. Creating all of that will take a long time no matter how much money you throw at it. For ARM, that ecosystem is probably the second best in the world, and in particular Apple has been building it up for themselves for more than a decade since the first iPhone. Additionally, sharing the ISA means that the ARM reference cores will always be available as a backstop if Apple's custom chips start running into trouble for whatever reason. IIRC, Qualcomm was doing a lot of hopping between full custom cores and ARM reference ones depending on which one happened to work better some particular year.