Ok, let's go back to your initial argument: 2W display. 500mW CPU.
Those 500mW reduce the use time of your gadget from 10 to 8 hours. Or from 8 to 6.4 hours.
When the media reports on tablets, one of the things that are always prominently reported are the 10h on iPad vs 6h on Galaxy Tab vs the rumored (and denied) horrible use time on the Playbook.
You don't reduce power with gigantic gobs at a time. Once low hanging fruit has gone, it's hard, tedious work and you have to fight for each mW. You can bet that integrators take notice when you're able to shave 50mW from a common use case.
In a vacuum, sure. But I noticed you chose specific examples. What if the design decision involved 12 hours vs 10? What if for that 2 hours of battery life you give up, you get something for it? Like a more powerful GPU or faster memory? Hell, how about it just cost more?
At some point, lowering the power of the SoC becomes an afterthought compared to other areas to focus on. Nobody out there is set (or will realistically) build the absolute perfect device. At some point, you'll have to prioritize.
Also, if you add in the 2W for the RF radio, 1W for the memory, 1W for the flash, etc. That number stops being 10 hours vs 8 hours and becomes 10 hours vs 9.2 hours or some change. Your assumption also totally eliminated SoC power. In reality, we can at best expect something like 350mW vs 1W and only as peak consumption.