I have heard about ARM chips and Apple eventually using those. As far back as 2011 or 12. Probably earlier but I know for sure during that era, there were talks of switching from Intel to ARM and what that could mean for Apple computing. But it also seems Apple is trying to kill the computer and get everyone situated with an iPad or Pro. People say it'll never happen, but that's not going to stop Apple from trying to lasso in casual users in to using an iPad and letting hardcore, heavy users stick with their computers until a tablet can handle those tasks as well. So like, ten years? Less?
ARM chips aren't as capable as x86 chips, which I assume is the main reason Apple uses Intel for desktop and laptop. Also Mac software would have to be rewritten for ARM chips, or suffer huge instruction translation penalty like Qualcomm chips do with the few Windows programs that work on them.
Also no matter how powerful tablet chips become, they are always going to be low powered, with heat dissipation limitations compared to desktop computers or even laptops. We are talking about 4.5W of power/thermal design power that they can handle. Laptops are comfortable with 15W-35W chips. Desktops usually have ~100W chips. While the performance does not grow as much as the power differences would suggest, even if the mobile chips grow to handle most software at acceptable levels, they are still going to be puny compared to true desktop chips. That difference grows even more if you factor in desktops using discrete graphics cards. PC gaming is still a growing industry, with more gaming PCs being sold than ever, although one can argue it's the area Apple had never covered.
On a side note, did you see the performance hit to the iPhone 6 and older with the new security patches? They took a 40% performance hit, which is insane, and makes people's older iOS devices basically obsolete for many tasks that their users could comfortably perform until now.
https://www.gsmarena.com/iphone_6_takes_massive_performance_hit_after_spectre_patch-news-29124.php
Something makes me think the security issues couldn't have come at a more convenient time, and solved all of their problems in terms of encouraging people to switch to newer devices. Especially as newer devices didn't take anywhere near this kind of a performance hit. In case anyone wondered why, Intel's patches also slowed down its older chips much more than anything with a Skylake onwards. It's easy to design such patches and get away with it too, considering everyone recognizes the security issue that "had to be" patched, and it's easy to justify also putting less effort into patching older devices, and making it seem like people should be happy they got a patch at all.
In the meantime, while the Meltdown and Spectre made Intel chips take a major performance hit, its Management Engine still being vulnerable, there is another vulnerability that affects Intel processors, this time related to their Active Management Technology, that allows hackers to bypass system encryption and BIOS passwords, which were regarded as the holy grail of security:
http://www.tomshardware.co.uk/intel-amt-bitlocker-bios-bypass,news-57714.html
Of course, AMD is not affected again. Moreover, AMD released a statement saying that their CPUs are by design not vulnerable to Meltdown or any other of the recently announced bugs except Spectre, which they just patched at no performance penalty, while Ryzen is not even affected by that, as its branch prediction units are protected against such things by design.