The latest models require repeatedly disassembling the sandwich from both sides. You have to remove the keyboard to access a screw, open the bottom, remove another screw to lift the top, then go back and forth between both sides just to reach a fan that frequently gets clogged with dust and needs frequent, regular cleaning.
Even worse, the unavoidable cleaning voids the warranty.
Thinner devices need more solid enforcement to stay stiff. My Thinkpad X60s weighs the same as a Macbook Air and it's infinitely more upgradable.