Modern smartphones and tablets are getting slimmer and slimmer, to the point that 7mm in depth is not seen as unreasonable. With the clever use of high-strength materials, companies like Motorola have managed to create products that are still as strong as others that are much thicker.
Motorola is the exception in many ways because it has managed to create the RAZR which offers excellent strength alongside a battery performance which is higher than you would expect given the size of the device and battery. Most smartphones rely on design above function and the relentless focus on making phones slim is often at the expense of battery life. If modern day-smartphones were 13mm thick, they could produce twice the battery performance and become much more practical and indeed stronger for day to day use.
The fact is, however, that we want our phones and tablets to be thin and light, and they will continue to reduce in size until the minimal depth becomes impractical. A phone can only get so thin before it becomes difficult to use and 5mm is likely the minimum that is practical. We remain to be surprised though at what the future holds.