(my emphasis on tools that are down a lot)
Huh, that's something I hadn't thought about before... If I catch the idea correctly: perhaps what you mean to imply is that there's a bit of a bathtub-curve going on, where:
1. brand new tools have higher failure rates not only because they're a new design, and they may have infant-mortality failures, but also because the fab staff needs to learn how to get them working correctly on-site
2. after a learning curve, the failure rate is lower
3. after a LONG time, the failure rate increases again, because the equipment has exceeded its design life
Perhaps in other manufacturing industries it makes sense to spend the money to reduce failure rates, but in semiconductor manufacturing the capital equipment is just so expensive, and first-mover advantage is so high, that it makes more economic sense just to spend the labor costs to keep brand-new leading edge tools up and running properly.
And in the case of mature fabs, it also makes economic sense to spend the labor costs to keep old tools up and running properly rather than to purchase new equipment and incur depreciation costs.
I'm probably missing something, but as someone who doesn't work in manufacturing, that's what I read into it. Am I close?