I think they could make it work if each was part of a 'complete' set - i.e. you buy the Dark Sun Box Set, you get the Dark Sun phb and mm and dmg, one 1-20 campaign, and some minis and a map or two. The trick is to assume that WotC will never expand on this - that's the Dark Sun product. After that's out - they're done making official Dark Sun products. If you want to homebrew stuff for it, pull in stuff from other products, by 3pp, that's all on you. Like Legos, they're you're to play with however you want and can be combined with any other product line, as you see fit, but the company isn't going to get into that. Just do your thing.
The next product from that team is the Official Spelljammer Box Set. (Ravenloft would come before Dark Sun, but I digress).
Of course they'd need to sell the Dark Sun PHB as a separate product, and might as well license a line of player minis, but the main thing is to treat each as a singular product so they're not splitting the brand.