- Then the other Astromech poops out conveniently so that R2 can be purchased
This isn’t a plot convenience, it was dramatic intent. Owen could have purchased both R2 and 3PO and the scene would have been over, but it would have been less effective. After the droids went through so much and were separated and then reunited, the possibility they might be separated again pulls on your heart strings a little bit.
This argument can (and should) be applied to TFA (and most works of fiction).