If every user is collecting 36mb a day like in the story here, your droplet wouldn’t even be capable of serving 500 users a month without hitting your bandwidth limit. With their current rates, your one million requests would cost you around 10 million USD.
That's ridiculously big quantity of data to serve a seldomly updated blog just because the client doesn't want (or know how, or think about) to implement an easy and old http method.
Imagine the petabytes of data transferred through the internet saved if a couple RSS clients added that method.
Yeah, but also... if RSS readers behaved correctly, it would be 512 kb. (170 kb with gzip, if she didn't enable it like you imply – I'm too lazy to check, but I assumed it was on.)
I think making clients behave correctly is much more sustainable solution, although we could do better than doing so at the cost of the end users.