Apple's current ARM cores are highly optimized versions - they are not using ARM IP off the shelf and stuffing it in a design. The operating systems are already somewhat converged. The big powerful ARM-based CPUs are there - AWS offers ARM instances. Check out the Ampere Altra line, right now up to 80 cores with 128 coming, and at certain core counts, manage to blow away any X86 in terms of performance per watt.
What even IS x86 any more? So many instructions just keep getting added. Extension this, extension that. Some tiny incremental IPC improvements with each generation, some, like Intel constantly requiring a new socket, AMD at least holding on to the same socket through a few generations. PCs are ALREADY faster than most typical users need. The outlier high performance cases will always need specialized hardware to optimize their tasks, but there's nothing preventing ARM architecture from supporting large floating point or fast integer calculation optimizations.
Apple already builds phone and tablet SOCs that regularly outperform the competition. And not just by a little bit. They have a rather talented team assembled that does this design work, easily on par with what the others have. There's no reason why, freed of the ultra low power envelope of a phone and put in a larger laptop or desktop where active cooling is possible, the performance won't be there.
It's mostly a given without an official word, that the real high end stuff like the Mac Pro will probably be the last to transition to an ARM system. But they blew it on that one from the beginning, simply not wanting to get into certifying the OS for a different CPU - an AMD Epyc in that thing would both double the cores AND be significantly cheaper. And this is where it all comes together - control of the hardware means a much tighter integration with the OS. Windows issues STILL are more with drivers and strange hardware than anything. That Windows can run on Apple hardware doesn't say as much about Apple's hardware as it does about Windows' flexibility. That flexibility, and requirement that Windows needs to run on any crazy combination of hardware you can think of, remains the Achilles heel of Windows. Excluding discontinued and used items - just look on a well-stocked PC hardware site and see how many combinations of CPUs, chipsets, motherboards, and video cards there are. Windows has to work on whatever electrically compatible combination of that stuff you can come up with. Mac OS doesn't have to deal with that. It's far easier to write something to work on a few dozen hardware combinations - all of which are actually somewhat similar - than it is to write something that can work on millions of combinations.