Everything you've named that you care about is directly related to the CPU or it's performance
1. Retina Displays require enough graphical display power. in most Apple computers, this is powered by the IGP which is built into the CPU package. A new CPU will = better graphical display performance for that retina screen.
2. Thunderbolt controllers are moving to the package too. And to use thunderbolt 3, (or even 2?) new CPU's are required.
3. USB runs better with a better CPU since USB has CPU overhead. Kaby Lake will eve ncome with support for USB3.1
4. More ram is a by product of newer CPU packages that support higher ram amounts.
the list can go on. There's a reason why it's called
CENTRAL processing unit. your CPU controls everything your computer does. Getting faster isn't the only thing new CPU's bring. You get better power usage. Better battery saving tech. better thermals for heat dissapation, more controller support, such as Thunderbolt or native HVEC h265 decoding.
so while overall the CPU speed change alone isn't going to be the biggest change from updating the CPU's, there are far more benefits than just that.
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Same. I can see the MacBook air being cancelled soon. The rMB will likely take its spot in the lineup as the thin and light. And the MacBook pro gets a refresh.