You can make that argument both ways - if you'd learned Fortran-77 and nothing newer, your knowledge would be obsolete as well.
Either way, you have the foundation, should you wish, to be able to bring it up to date without too much hassle.
I think you'd be surprised how simple an ARM actually is under the skin. Admittedly, it's not Z80-simple, but it's far, far removed from the x86/AMD64 monstrosities.
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You can make that argument both ways - if you'd learned Fortran-77 and nothing newer, your knowledge would be obsolete as well.
Either way, you have the foundation, should you wish, to be able to bring it up to date without too much hassle.
I think you'd be surprised how simple an ARM actually is under the skin. Admittedly, it's not Z80-simple, but it's far, far removed from the x86/AMD64 monstrosities.