The problem is relatively new in mobile history to have heat problems. I have had phones since the 80s and in the last 10 years have been a contributor on certain Android forums and its only the last year or so has heat shot up the popular topic list. I think technology is the problem. This is because any usage builds up heat. More power CPUs generate more heat when working hard. After a while, to prevent damage the phone will slow to reduce the temperature. In a PC, as processors became more powerful the heat could be controlled by fans and even watercooling but for phones the technology has not changed. All of the components are close to each other and contained in a small sealed environment with nowhere for the heat to escape.
I think we need technology that will address this issue as it will get worse if cpus continue getting more powerful.