Why? A serious hobbyist photographer (for example) could easily generate that many legitimate files. S3 isn't a reasonable alternative for them.

S3 is expensive per byte, and your use of "that scale" confuses me for an amount that's only about 15% of a new hard drive.

Despite some occasional competition between 3+ of them, all of the large cloud providers continue to treat 1TB like it's still yuge - and something that only a for-profit enterprise willing to spend hundreds of dollars a year (or more) on storage may encounter.

I was more thinking about the quantity of files

Save some web pages and install some portable programs into the folders you want backed up and the number of files adds up fast.

Isn't Dropbox basically a sync client for S3?

Dropbox now uses its own data centres.