There are two kinds of enhancements to consider: Enhancements that are specific to your product and enhancements that are general to the library you're incorporating.
If Rails had been AGPL, every single webapp based on Rails would have to have its source code released. Now, Rails is not AGPL, does that mean that anybody using Rails (and not making their entire project open source) are "useless"? The vibrancy of the Rails community seems to disagree with that assessment.
I'd argue that ffmpeg belongs to the platform in a YouTube like usage scenario. Compare with ActiveRecord in Rails: Everything single thing your apps does depends heavily on it, yet, it's still just a piece of infrastructure.
It boils down to interpretation of what constitutes a "derivative product". I think it would be better if whoever used Rails and made changes to Active Record _had_ to share these changes (a-la AGPL), without having to share the app they built on top.
Similarly, it's better if ffmpeg users who offer a service based on ffmpeg were required by the license to share their ffmpeg modifications (the way Google already does without being required to) without a need to share the rest of the webapp.
> I think it would be better if whoever used Rails and made changes to Active Record[...]
But doesn't it at this point boil down to an academic argument? Surely Rails benefits massively from being popular, and draconian (which I consider AGPL) licenses might hamper its popularity. I doubt that Rails misses out on significant contributions just because they are not required. Contributing to open source looks good on a CV/employer branding page.
