If you want a RAID array on a Macbook series laptop, you can get a cage to replace the optical drive and pop in a second 2.5" drive of your choice. It's a compromise, but it's doable.
Not sure that CPU replacement is -that- common an upgrade, even for desktops. Every few years, the latest and greatest CPUs seem to use a new socket anyways.
It depends. I bought an i7 when they first came out a few years ago. My motherboard is still compatible with the newest i7s; if I wanted a 6-core CPU, I could pop one in right now.
Not sure that CPU replacement is -that- common an upgrade, even for desktops. Every few years, the latest and greatest CPUs seem to use a new socket anyways.