I'm wondering about whether I should replace my 36t middle chainring on a 46/36/26 triple. Standard 5-bolt 110 BCD, but that doesn't matter here. What does is that I use the middle ring for the vast majority of my riding, and I'm worried that it's worn out and needs replacing for the second time. I replaced it two years ago.
When I line up the teeth with teeth from a brand-new 36t Sugino chainring, the gap is a bit wider, and I can tell that it's because the chain has worn down the teeth a bit. (And yes, I understand that these chainrings have different tooth profiles to aid in shifting, and I'm not mistaking that for wear.) What I'm wondering is, how much are the teeth worn, and how should I decide on whether to replace the chainring?
It's clear why and how a worn chain is a danger to wear out a good chainring. It's less clear to me how much a slightly worn chainring is a danger to wear out a new chain. (Basically, I'm thinking that if all teeth are equally worn, then a non-stretched chain will still be equally pulled by each tooth with which it has contact, so a new chain won't be worn out by an older chainring, at least not much.) So that's a general/theoretical question.
For specifics, here are four pictures of the chainring's teeth. The first two shots are from the inner side of the chainring, so the left side of each tooth is what pulls on the chain and is worn.
The third and four shots are of the outer side of the chainring, and in this view the right side of each tooth is what pulls the chain and thus is worn.
Again, this chainring's teeth are slightly worn, when lining them up with teeth of a non-worn chainring. The part that doesn't line up is the part where the tooth contacts and pulls on the chain link.
For comparison, here are a couple pictures of a very, very badly worn Campy 30t chainring. My brother rode 7000 miles, possibly a fair bit more, without changing his chain...