There's lots to be gained with a high volume disc with better compression that would not take up more space.
Let the marketplace determine the demand.
There is also a potential gain in using some of that space. You may not agree with it, but I know some others that do. And it's not unheard of based on some past releases.
Easy to say about the marketplace determining the demand. But someone has to take the risk and assume the loss if it doesn't sell well. I understand you would like it and would be willing to buy if done, I just don' think the market would bear that out and I don't think it is worth it to the studio's to put out status quo just to conserve the number of disks. The double dipping model has more traditionally been based on getting better quality and/or more extras as the incentive.
I agree with an early poster that the driver would be when the DVD format is retired, but by then physical media might be completely out. This of course is just my opinion.