Okay, first off, no one game can save a system entirely, so don't count on Kingdom Hearts doing much. This is evidenced nicely in Japan for the PSP by the fact that, while Monster Hunter Portable 2nd did spike a massive interest in the system, it has done nothing for overall software sales over there, and may well have just coincided with the DS' market saturation leading to higher PSP hardware sales.
Second, the drawbacks of using UMD far outweigh the benefits. The drawbacks being that UMD uses a heck of a lot more power than a solid-state medium like cartridges or flash memory, and that its medium (not being solid state) is both more fragile and more prone to eventual failure. Two things which make a handheld console look appealing to the average consumer are a long battery life and durable games, and UMD fails to deliver in both regards.
Sky Render - Sanity is for the weak.
