Can a trend of android phones slowing down after 2-3 years of usage be attributed to the low durability of RISC CPU used in them?
Android phones slow down because the software applications are rapidly changing. More features get added, and this makes the software consume more memory (RAM) and CPU time. Software keeps getting updated and each update results in it requiring more resources. In response to this, smartphone manufactures have increased the capabilities of the hardware they manufacture. RAM, CPU Frequency, number of cores are increased, and this increases the performance. Thanks to Moore's law, the hardware capabilities and performance tend to be increased without any noticeable increase in cost.
One more reason why smartphones need to be changed continuously is because the Operating System gets updated. Google and Apple allows users to update their Operating Systems by using the same phone only a limited number of times. After this limit, users have no other choice but to buy a new phone to use the newer operating systems. As the support of applications on older models of smartphones gets dropped, the user has no other choice but to upgrade to newer phones. This is known as planned obsolescence.
For x86, a similar phenomenon is observed, especially in Windows based laptops. As Windows keeps getting updated, you will observe that the laptop gets slower. This problem can be solved to some extent by increasing the RAM (which is not possible in the case of smartphones). For PCs, you can upgrade the CPU, GPU, RAM and storage, so they tend to be used the longest. For PCs, the Operating System can also be easily changed, and a lighter operating system can be used, which improves the performance.