When you can get a Gb of HDD storage for as little as 5p, it would seem daft not to get a huge HDD. A solid state makes sense on the road, as it reduces the strain on the battery. But I wouldn't buy a laptop without a DVD player/recorder. I can think of loads of reasons for one on the road.
Also, we aren't really limited by the keyboard and screen. Apple have shown just how thin they can make a screen, and a keyboard isn't really that thick. I think you are forgetting all the other parts that a computer needs to run, most of all the battery, which, with every laptop I've seen, tends to take up a fair share of room. The bottom line is, yes, you could have a thinner laptop if you removed the DVD player, but I would rather just put up with a few mm's extra thickness to be honest.