Decoders also have the difficulty that you need to support most of the format’s features before they can support content found in the wild. Also you often need to add hacks to support encoders which technically violate the specification but are commonly used.

In contrast, you can build a very simple encoder using very few of the format features and still have it be usable/useful (albeit with poor quality/compression ratio).

Not many.

Weirdly, chatgpt can be remarkably good at translating code between programming languages.

I suspect within a year or two it'll be pretty easy to translate a lot of C libraries to native rust code (or whatever) using modern AIs.