it doesn't give a sense of how much the Neo moves the needle or grows the pie though. Mac and iPad are each 8% of Apple's business. iPhone is 50%.

for Apple to focus on fixing things in macOS again (as opposed to just backporting stuff from their other lines) Mac would need to grow by a lot.

Not really. An independent Mac company with 8% of Apple's sales would still be the #3 computer manufacturer behind Dell and HP, and Mac gross margins could easily support significantly larger investments in OS development if Apple chose to do so.