What is missing from these discussions is what our personal tolerance is for the manifestations of chain and cassette wear. I swap out a chain when the shifting begins to be anything but perfect. I love the exact crispness of a good quick shift. That is just my preference. Others may be more tolerant of shifting that isn't as precise. If I was riding downtube or barend shifters, maybe I could also get 10,000 miles because I would have more direct control over the shifting with those systems than with any indexing system.
Since you brought that up... As one of the folks who tends to get very long wear out of chains and has indexed shifting on all of my bikes, I have not found that within the normal range of chain wear that shifting is affected much as the chain wears. I find that once I set limit screws on the derailleurs they never need to be adjusted again, but cable tension does need a tweak once in a while on indexed systems. That probably means tweaking it a few times on a coast to coast length tour and usually amounts to twisting a barrel adjuster 1/4-1/2 turn or so, which I typically do while riding.
I typically check my chains by measuring 12 complete links. When they are 12-1/16" it is time to start thinking about a new chain and I make sure to get one before it is 12-1/8". As I said, within that range, it has never seemed to me that chain wear had anything to do with any shifting issues.
I find that cassettes and chain rings typically last me a very long time. So long that I have no idea how long. I have found that cassette wear and derailleur wear have a lot more impact on crisp accurate shifting than chain wear. A gunked up drive train also has a negative impact, but with my current routine that doesn't usually happen on tour. Gunk build up does seem to be more likely for commuting for some reason I don't understand.