If the provider of proprietary software ceases maintaining it, you are dead in the water. If the provider of open source software ceases maintaining it, you can pay someone else to maintain it. E.g. there are a lot of freelance jobs related to ffmpeg, a lot of video software experts knowing it, so you may sleep well, not worrying that the developer of ffmpeg will someday sabotage your business. On the other hand, when another company provides a binary blob library to you, when they go down, you go down with them (if you rely on them too much).