There wont be double layer UMD's that's technically impossible

Of course things like compression and such will get better over time. Games will also look nicer; everything will be better within time. Soon we will notice that games are longer, look nicer, and load quicker; but it's all relative as to the game developer's budget on the game, as well as the game developer itself (whether it's and Electronics Arts production or some crappy independent Australian one).