Apple license the instruction set and make their own designs, so the interest in all that overhead business, let alone the political fallout (too much power in one company and fuels the Apple TAX headline mantra that will just roll and roll) - Apple don't want that hassel. But many reasons Apple buying ARM is not a good fit for Apple. Nvidia may well be good fit and a better fit than Apple by far, though would it be…
ARM is definitely mature, it originally IPO'd in 1998 and remained public until it was bought by SoftBank in 2016.