My guess is that they'll just say Safari isn't supported and push people to Chrome.
Let’s say ~30% of Etsy.com views are through Safari (iOS + macOS; ignoring that Chrome on iOS is a viewframe around WebKit anyway). Of those, 25% have iCloud+.
Let’s say they did this and 50% of people did visit the site through Chrome.
That’s ~4% less revenue for them. I don’t know what Etsy makes per year, but ~4% of whatever that is will be on the order of millions of dollars. So, this is a no-brainer.
Let's say that ~100% of Etsy.com views are through Safari. Of those, 100% have iCloud+.
Let's say they did this and 0% of people visit the site through chrome.
That's 100% less revenue for them. Pretty amazing that this one feature could completely kill Etsy's revenue stream.
(For what it's worth, I agree with you that Etsy is not going to tell Safari users to pound sand, but your arbitrary numbers don't make an actual point unless they're based in facts and figures.)
In other words, your comment reads as “if everyone stops visiting Etsy, then they will make $0”, which…yeah. Makes sense to me.
They plug the numbers into the formula and see "If we block IE11 users from being able to use the site, we lose 1% of our traffic, which equates to X dollars. Is that a substantial amount? If so, we support IE11, if not, IE11 support goes out the door."
So yeah, you can plug in the numbers you did and try to negate his argument, but that doesn't make his argument wrong. Just means you understand the argument but fail to accept it.
2. The point of the math was to show that it's a big deal for basically any reasonable values. It doesn't depend on the exact numbers.