I guess I'll admit that fixed width fonts are a valid use case, but I'd argue that that defeats the point of displaying language to humans. There are so many cases this doesn't apply: RTL languages, kanji, pretty much anything that's not eurocentric.
My original response was going to be that you don't actually care about "foo".length in that context, either. You care whether one string is larger than the other, i.e. the actual value doesn't matter -- all that matters is that if a string X is longer than string Y, then the length should increase. But mathematically, that's just a precise definition of requiring the string length, so that logic doesn't work out.
If you email me your paypal/venmo/whatever, you can collect the $20, since you were the first to point out monospaced font. I still feel that monospaced fonts are sidestepping the whole point, but I won't be pedantic about it.
(https://news.ycombinator.com/item?id=29303325 points out why this isn't a sensible operation, if you were curious about why I feel inclined not to concede the bet.)