Indeed, such alliances can be fragile, but I think this one will last. At least, I think its fragility is the fragility of medium format, not of this alliance. Why? Because they have no one but each other. Hasselblad took away a major source of sales from Phase One when it decided to "team" with Imacon, and make only dedicated cameras and backs. With Leaf buddying up to Sinar, there really isn't any one else for Phase One to sell to (well, there's still Rollei, of course). And there's no way for Mamiya to effectively continue without a good digital back to work with its cameras. It's ZD was a good try, but it can't keep pace. I certainly hope this union lasts, and works. Phase One backs are, from what I read, as I can't afford one myself, very good digital backs. Many think they are the best. And I've always liked Mamiya. My first camera was a Mamiya Skeor 35mm.
As for competition from the DSLRs. Yep, that's the real threat--not just to this alliance, but to medium format in general. There are advantages to MF. Because the chip is physically bigger, the photosites can be bigger, and the images less noisy, cleaner. And of course the MF backs are now at 39 megapixels. Michael Reichman at the Luminous Landscape suspects there will be 60 megapixel MF backs within a year, and even larger sizes down the road. Since most people also think the 35mm DSLRs won't get much bigger than 25 megapixels (who really knows, though), 60 or more is a significant bump. They will likely always be a niche market--for the photographer who needs absolutely the highest resolution in a single shot and cost be damned (I think that's pretty close to a Reichman quote). At any rate, they may survive. After all, people still buy scanning backs.