Ignoring that Apple's chips as laptop flagships have pretty much singlehandedly changed the perception on their viability for actual computers, how much of ARM's work are they actually using?

They design their own chips that are meaningfully ahead of ARM's. I understand that their contract allows them pretty broad access to IP, but are we sure they'd be that much worse off (especially compared to ARM) if this deal wasn't signed and Apple put the investment into a different instruction set?

Hell, they built most of the smartphone and tablet market. Are we sure ARM would even be relevant without Apple's weight?