This assumes that there isn't some other factor in transitioning architecture - this argument could boil down in the mid 2000's to "Why not go x86/amd64", but you couldn't buy a license to that easily (would need to be 3-way with Intel/AMD to further complicate things)
Apple has done quite well with their ARM license, outperforming the rest of the mobile form factor CPU market by a considerable margin. I don't doubt that they could transition - they've done it successfully 3 times already, even before the current ARM transition.
Apple under Cook has said they want to "to own and control the primary technologies behind the products we make". I doubt they'd turn away from that now to become dependent on an outside technology, especially given how deep their pockets are.
But that has really nothing to do with the architecture. They just spent more money on CPU R&D than their ARM competitors. They could have done the same thing with RISC-V, and if that's where the rest of the industry is going, they could be better off going there too. Especially for Mac, since they're about to do a transition anyway. They could benefit from putting it off for another year while they change the target architecture to something the rest of the market might not be expected to avoid in the future.
There is also no guarantee that their success is permanent. They might have done a better job than Qualcomm this year, but what happens tomorrow, if Google throws their hat into the ring, or AMD makes a strong play for the mobile market, or Intel gets their heads out of their butts, or China decides they want the crown and gives a design team an unlimited budget? There is value in the ability to switch easily in the event that someone else is the king of the mountain for a while.
> I don't doubt that they could transition - they've done it successfully 3 times already, even before the current ARM transition.
Just because they can do it doesn't mean it's free.
Really if Intel was shrewd, they'd recognize that they've lost Apple's business already and just sell them an x86 license, under some terms that Intel would care about and Apple wouldn't, like they can only put the chips in their own devices. Then Apple could save themselves the transition entirely and do another refresh with processors from Intel while they put their CPU team to task redesigning their own chips to be x86_64. It would give both Apple and Intel a chance to throw a punch at Nvidia (which neither of them like) while helping both of them. Apple by avoiding the Mac transition cost and Intel by maintaining the market share of their processor architecture and earning them whatever money they get from Apple for the license.
And it gives Intel a chance to win Apple's business back. All they have to do is design a better CPU than Apple does in-house, and Apple could start shipping them again without having to switch architectures. Which is also of value to Apple because it allows them to do just that if Intel does produce a better CPU than they do at any point in the future.
And you don't want to move to ARM if everybody else is moving away from it.
There is one thing they would need to worry about though, which is that if the rest of the market moves to RISC-V or x64 or whatever else, it's not implausible that someone might at some point make a processor which is superior to the ones those companies make in-house. If it's the same architecture, you just buy them or license the design and put them in your devices. If it's not, you're stuck between suffering an architecture transition that your competitors have already put behind them or sticking with your uncompetitive in-house designs using the old architecture that nobody else wants anymore.
Their best move might be to forget about the architecture license and make the switch to something else with the rest of the market.