Note that the biggest customer/developer pain from the PPC->Intel and Intel->AS transitions are not the CPUs themselves per se, but the ISA changes.
Apple's current arrangement is that they license the rights to the ISA, but have the freedom to do whatever they want in their CPU designs. And they do. They have their own "from scratch" ARM CPU cores much in the same way AMD has "from scratch" x86 CPU cores.
Being able to leverage an existing ISA, but design their own architecture backing that ISA prevents iOS from going through one of these ISA shifts when Apple started replacing reference designs with their custom designs, so it makes sense for Apple to license the ISA like they do. Any future ISA shift will very likely affect the Apple's whole product line, so there is a bit of a disincentive to do one so quickly after migrating the Mac. It's not exactly cheap on Apple's part to provide test hardware, transition bridges like Rosetta, burning goodwill they've built up, etc.
I'd be surprised if Apple switches ISAs anytime soon, though. While they don't need anything from ARM, they have a very practical arrangement that has benefits for Apple. So Apple does have an interest to prod future ISA additions and changes in a direction that benefits them as long as they can maintain a good working relationship.
Why do they need "Arm Cores" ?
If they don't need ARM and design their own silicon genuinely from the ground up, then they don't need to copy? anything from ARM whatsoever do they?