The issue isn't merely the same vendor, but having to buy parts bundled together. Good luck replacing your hard drive/RAM/whatever separately from the rest of your machine without blessing from the manufacturer.
The upgrade path is to throw away the entire machine and buy a new one.
I've often wondered why the boundaries were defined where they are for its responsibilities. What was it like building PCs before the concept of a the current motherboard architecture existed?
Maybe the solution is to getting people to repair desktop motherboards instead of tossing them is to turn the cost to the environment of generating e-waste into an actual monetary cost. (Though I can't say I know how you'd do that since making e-waste disposal cost money will mean that people will just dump that stuff in the normal trash)