That was a problem I actually had when I had no budget, was buying old parts, and then running them way longer than they were intended. I kept everything clean, the tower wasn't on the carpet, and there were no smokers or pets shedding fur, but that PSU eventually started outputting significantly lower than it was rated for. The previous owner could have done something to it, or it could have been a crappy model to begin with, but it was about fifteen years old and I was told by several more veteran computer folks that PSUs would drop off in power output eventually and this wasn't surprising.

If you are filling your PSU with tar from cigarette smoking, yes, its max wattage will go down over time.

It's like making the marathon runner inhale your smoke while running the marathon.