The primary tenent of the web is the robustness principle: be conservative in what you do and be liberal in what you accept. Graceful fallback to a static image fits that principle.
Exactly. Many formats (PNG, HTML, etc) were designed with that goal in mind. Old browsers are able to skip/ignore any new HTML tags, and old PNG readers know how to skip/ignore new stuff (like APNG chunks) and continue parsing till the end of file.