As long as 'free for personal use' means 'closed source' then at least I would have a hard time trusting a tool backing up personal files into public cloud storage without transparency about how the crypto is implemented. I suspect that may also be the case for much of your 'personal use' target audience.
A thought: Do you think people would pay collectively (given the chance) like a fundraiser to publish the source, when a certain limit is reached? Maybe even pay the developer's bills that way? I wonder..