Not to mention not everywhere in the world has unlimited transfer limits on their internet. Example most ISPs in Canada have a 20-250GB/month cap counting both downloads and uploads. Afterwards you pay about another $100 a month before the penalties stop. Even if they don't directly penalize you you likely would be paying for a higher internet package/getting traffic shaped because of your high usage. So your free/cheap backup will likely cost you several hundred in penalties to backup and several hundred more should you need to restore making it almost as cost effective to just have a second 20TB array.
I get people like to do this but the easier solution for 99% of people is just to get some counseling for their hording behaviour. How likely are you t watch that S3E04 Gilmore Girls episode again? I went to university with a guy back in 2000 when storage was relatively expensive that had 1000's of anime movies. He hadn't watched most of them and didn't really intend too. He downloaded them and then if anyone recommended one of them to him he would watch it. We was downloading: just in case one day he might have a reason to be interested.