[Xperia Z3 Compact Q&A, Help & Troubleshooting] 2mal16:
Well most of these question can only be answered with thorough testing and comparison of lets say "affected" phones , phones without the water seals etc... while keeping the components and architecture the same.

Batteries are disposable products so why should companies follow up on this? Except for the cases where its really a hardware problem, like when your phone catches fire or something they wont care about this kind of degradation.

Generally you can say the more power the more heat development. Though the degree of this again is entirely dependent on the components physical architeture, material and the applied cooling solutions. So every smartphone will be different as regards heat development.

This phone is still too young to speak about the degradation of battery capacity. But modern batteries tend to do a fine job. Heat again reduces battery capacity and component life span so if u tend to walk around with your smartphone always recording in 4k it will live shorter. But that is kind of obvious.

Processor power does not really degrade. There is a point when it will die due to wear of the material but a processor either works or it doesnt. Hence a damaged processor would result in a non functional phone. Also the processor, if done the right way, will outlive other components in your phone so i would not worry about it too much.

What can be damaged as regards processor is the cooling. If it is not cooled down properly it will overheat, shut down and eventually be rendered damaged and unusable.

The two items that when looking at computers generally wear out are the harddrive (SSD or HDD) and the battery (as regards laptops). SSDs tend to have a huge amount of lifespan nowadays so battery is and will stay the weak point.Thats why more and more development is put into power saving features rather than pure power. Just look at the development of Intel. Processor power has not gone up much in the last 6 years, but power saving has. A smart move bcs more and more ppl buy portable devices instead of stationary workstations.

So yes , after two years of constant usage you may be down to 60% capacity of the battery.

Usage of battery is also said to be important, but here is where i tend to say just use it how you want otherwise it gets too complicated :).

If you want to use your phone longer than that, without having to live with more frequent charging, you will have to buy one with a replaceable battery, get an additional battery pack like the mugen power or exchange the battery yourself. There is just no other way.

The trend of no user replacable batteries is a bad one. Yet profitable for the companies. You can see the same in laptops, only some of the top tier models like expensive lenovos tend to have replaceable batteries so they can last longer. Phones and Laptops are getting thinner and thinner (eg ultrabooks) which is used as an excuse that batteries have to be integrated into the case, leaving the (normal) user with no access to it. This is all a lie and just pure money making.

A solution for longer longetivity can also be that you cap the performance of your device. So reduce the maximum frequency of your processor, undervolt it and stuff like that. It will decrease heat development and battery usage but of course also decrease performance. But then you dont need the Z3 compact, which purpose is exactly that it is a very capable phone with a small footprint ;)

at last here:

a generally good review site is this one http://www.notebookcheck.net/Sony-Xperia-Z3-Compact-Smartphone-Review.127925.0.html

if you scroll down they always test heat development of the devices. So you may get an idea of longetivity, if lets say heat development is unusually high.