If they tried to go that route with Adobe, they'd get sued for antitrust on the spot. Their second option, of course, is what they do with Office: continue offer the product, but let it lag behind, and make several features (such as collaboration) available as Windows-only options. Basically, a lobotomy.
Apple may have made people unhappy with what they did with Logic Pro and Shake (which got cannibalized for Final Cut Studio and then discontinued), but they weren't buying the only game in town; Windows is, after all, famous for software choice, right? Adobe, on the other hand, is a monopoly in the creative market, and its discontinuation on the Mac platform would be an unparalleled disaster. The legacy of Photoshop overshadows even that of Office; there simply is no realistic competition at this point, more's the pity, and even if Apple stepped in and tried to fill the gap, it'd take several years for a realistic replacement to emerge - that's simply not possible. We'd be back in 1996 again, slowly bleeding users over to the Windows platform.
Having said all this, it's important to note that the current competition from Apple and Microsoft's point of view is the mobile space. Adobe would be insane to let Microsoft buy them out, as Flash still comes in second to the importance of Creative Suite to their bottom line. Microsoft is not even a player in the mobile space at this point; they're going on name recognition alone. As good as their new platform appears to be, they also run the risk of the same kind of market fragmentation that Android now faces. Adobe can hitch their wagon to the software platform, but unless Microsoft takes a more draconian approach to hardware control with the vendors (which would make them serious contenders against both Apple and Android, and they of all companies should have the clout to do it), they're not going to any more benefit from the partnership than allying themselves with Google.