That's a really good point I hadn't considered at all; I'm glad you mentioned it! I was looking at a benchmark (that i linked in another comment) that makes duplicity look slow, but so much more economical on storage space - i.e. cheaper.

But as you point out, if you don't need a long history, incremental eventually gets more expensive. Unless you could squash older than X, I suppose, but presumably that's so expensive to run (encryption & compression) that it's not supported.