Re:My worry is... (Score 2)

So what, with 72x more data, you can afford to put 36x more information on the disc twice.
The point is that it's now 1/2 the size...

I also don't see you arguing against your hard drive becoming more and more dense. The same problem exists there.
But you don't handle the platter of the hard drive directly as you do with a disc. Apples and oranges.

Just use multiple lasers to read the data, that should be able to correct for many scratches, and movie/music data is less critical than computer data, error correction is built in, and a good CD/DVD player should be able to either find, extrapolate, or simply skip over the data that's missing without us noticing unless it gets really severe.
Good point for movies. Also, the increased data used to display each frame would mean that even if a given amount of data is lost to a scratch and ignored, the actually amount of frames would be less (MB to MB). For critical data, redunancy would be required.