Why can't Apple allow/sell basic upgrade proprietary hardware?

If Apple would market their own basic hardware upgrades through Apple & their re-sellers, they would create a new, BIG revenue stream by allowing consumers that might not be able to afford to 'buy new' as soon as others at least 'upgrade' & stay loyal customers? Sure makes a lot of 'common sense' if you look at the 'BIG PICTURE' for Apple & their partners. Just don't 'overprice' basic hardware (like ram) upgrades because these are the consumers that can least afford it if they can't buy a whole new product. What it does is keep them a happy, loyal future whole new product buyer ;-) PLEASE consider doing this, Apple. You're missing the boat otherwise...Jim