Regarding the components in the belt path:
I just replaced the timing belt on my car, at 301,000 miles.
I have documentation of it's last replacement at 224,000 miles, and it had the "100,000 mile" kit of parts installed, with a Hepu pump, Ina idler rollers, and a Littens tensioner if I remember correctly.
As I took all the pieces off, I was inspecting for the reputable brand names that I expected to see, and did find present. What I also found was that even with only ~80,000 miles on the timing belt components, there was some detectable wear and slop to the bearings on a few components.
One of the small idlers had a barely felt wobble to it, if you hold the core and rock the outer surface, while the other felt fine. The large idler felt a little more loose yet.
The serpentine belt tensioner roller was notchy, the dampener was dead, and the idler roller was sloppy loose.
Personally, even though I just spent ~$500 on parts for the timing belt, serpentine belt, and alternator pulley (which had also seized hard), I will be planning to replace the timing belt in another 80,000 miles, and inspecting the serpentine belt path earlier than that.
These are complex machines, and things do wear. That said, I don't feel like it owed me anything at that point, as the car has already done 300,000 miles, and gets great fuel economy while doing so.
Doing the same job on my last car, a 1984 Mercedes with a v8, was still a 100,000 mile interval, and if you had to go beyond the chain and tensioner, you were talking about tearing the heads off to get to everything, which was hidden down in the timing gallery which spanned the whole face of the engine. (It also only got 15 mpg on average, burning premium unleaded the whole way!)