The average lifespan of a CPU

Usage of a CPU does result in wear at the atomic/electronic level.

The actual lifespan of the silicon transistors of a consumer CPU is typically in the range of 20-30 years before there is a failure, not 3-4years. It is asssumed by then that the item would be obsolete.

How does Intel/AMD know if there's no way to test a chip for 30years? It is tested under load under higher than normal conditions (heat voltage, clock) and the variables and failure data are then extrapolated and calculated backwards for typical use.

Of course there are other failure points such as the chip packaging wires and things of that nature, but low decades would be a fair assumption.

3-4 years is more of the practical obsolescence of the product due to Moore's law and all that.

Source: Masters degree in EE, and learned in a course on chip failure design.