The lack of hardware upgradeability on Apple products in general kept me from switching for many years. Then I realized by the time I was ready to upgrade my system the CPU sockets changed, the memory changed, etc.
So I'm not concerned if I can't upgrade my hardware down the road. Having everything soldered in place means I get a lighter product with fewer points of failure.
The more you play with a component's mechanical housing the more likely it is to break. There's also just human error in replacing hardware that leads to housing fractures.
Creepage among memory chips is also possible. RAM doesn't sit as snug for the entire life of your computer. It creeps out a little bit over time and allows for dust to prevent conductivity between the chip and the slot. Having everything soldered in place is much better.
The biggest issue, though, is all the added space required for the housing. The slots, connectors, clips, etc make the product bulkier. While that's fine for a desktop it's not ideal for a mobile device.
By the time I tend to want a new CPU it tends to be a new architecture anyway so I have to replace most of the parts. Other parts of laptops tend to be replaceable.
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.
And it costs more than a desktop + netbook combo.