the question is legit but remember there's FOREX where you trade currencies against each other. So why not sbd?

In esence, what SP gives you is not "interest". That's what the miners mine and send to you - procentually - and it's a protection against dilution [ 90%] the 10% you refer to needs to be made up by posting curating etc.

I am not sure that you'll ever go down because, as i said, it's not interest.