Sorry, but this isn’t neumorphism at all – by your very definition at the top, (giving buttons and UI elements highlights instead of drop shadows) – this isn’t happening on Big Sur. If anything there’s been a reduction in Skeuomorphism, look at things like window elements like Finder – all the heavy use of shadows has been stripped out.
Only place they’ve kept any sense of craft is in the icons, which has been part of MacOS legacy, and goes to show that they do deeply understand Mac developers.
On your point of sliders looking like they’re straight from iOS, why don’t you open up the music app on your phone and have a look at what slider they use. Visually the new sliders are a balance between the original ones and the new sliders in the iOS control centre, but don’t at all behave the same way.
On the ARM transition – Apple’s done this before when they transitioned to Intel, they know what they’re doing, this isn’t a windows RT situation.