What makes you believe the autonomous car project comes at the expense of Mac improvements, rather than being unrelated?
Yes you don't want to be a total one trick pony, but if you do everything, you do nothing well.
Sometimes that's true, but I think that most often, these kind of transactions (M&A, spin-offs etc) are the result of political gaming on top of companies, rather than driven by actual business or financial needs. Top managers play their own game very well, and at the end of those transactions usually there is a bonus paid out, no matter the results 5 years down the road. It's not as if verticals within a large company hadn't their own specialized staff, focusing on their own products.
Is this absolutely the reason the laptops are a bit crap? No, but it is plausible that the lacklustre offering in hardware comes from the lack of it being a priority for the company. It certainly doesn't feel like a priority for them, and the offering doesn't feel like a lot of strategy has gone in to how it has evolved over the last few years - contrast that with the iphone.
I wouldn’t touch a Mac laptop with a ten foot pole except for maybe the Air if I was desperate, but the iMacs and Mac Minis are tempting. I don’t do development anyway without at least two external monitors and my favorite keyboard/mouse combo.