Here's the problem with that: most people will use this (I assume) only once or twice, to, say, create instructions for their web app (like commented above), and would be happy to pay 20/40/60$ for the software, but there's no reason for them to buy a subscription. Unless you're aiming at an audience that creates instructions multiple times a month?
You obviously haven't heard of e-learning industry. These kind of tools are used very regularly by e-learning professionals, and instructors. Sometimes everyday. (How do I know this? I was doing e-learning for a few years.) But this tool faces a stiff competition from established and more sophisticated tools like Adobe Captivate, Camtasia, http://www.sameshow.com/
To add to this line of thought, charging per video made would be the way to go. You could limit the free version not only with the downloading and branding options, but also with the length the of the video (eg. max length of free video 30 seconds).