True, however, also software has evolved quite much during that time. Both in features and security, which makes it heavier to run. Some of that may be possible to improve with fine tuning software, but in current world it's usually easier and cheaper to get acceptable results by trowing more iron to the border. It's the natural result from quicker release of software.

Also, ie. iPhones have relatively small battery capaciy (as they are intended to be smaller and thinner devices while packing some serious SoC performance), yet they have decent battery life, comparable to devices with significantly bigger batteries.

That's only one part, the other part is (as I mentioned previously) shorter release cycles to bring new features to the market quicker.

This depends on what you are going to do with it. It's not reasonable to throw latest SoC in to just write text messages, but running underpowered SoC on some other workload usually means higher power consumption as higher loads last longer periods of time.

True, but in situation where SoC-manufacturers more or less hate native Linux, it's not viable in the long run. AFAIK there are not many relevant SoC manufacturers that offer native Linux drivers or even documentation needed to create open source drivers for them. In that sense it's unfortunate that ST-Ericcson falled apart, thatSoC would have had native drivers supported by the manufacturer.