Sorry, I don't get what you're arguing here. Are you saying the C-compilers generated poor code when the FFmpeg project was started back in 2000? Are you arguing that if the FFmpeg project had used Modula-2, this would have sparked more work on Modula-2 compilers?
Or are you perhaps just wishing that history had taken a different turn?
Compiler support across various platforms: are there compilers available and do they generate good code?
Mindshare: how big is the intersection of people who know the language and have the domain knowledge to contribute to the project?