> jpg and mp3 and zip are effectively “final” image and sound and data codecs

In popular consciousness, maybe. When people think image they thing jpg. But most websites you use aren't delivering images as jpg. Most sites care about saving money on bandwidth and their site loading fast, and because of that they tend to use webp and avif, which are much smaller for the same quality.

jpg: webp, avif, jxl. The first two are adopted because of non-negligible gains, the latter because of features missing in jpg.

mp3: Both AAC and opus have non-negligible gains and both are fairly widely used.

zip[1]: lz4, zstd, and lzma2 dominate at least gzip's implementation of deflate: For any given setting of gzip, there is a setting for at least one of those three that is both faster and smaller.

1: Not a codec; the zip format specifies a dozen methods for encoding. I'm assuming you mean Deflate, which is the most common compression method associated with zip.

Re: mp3, Opus is a pretty big improvement. In my experience I've found that Opus is pretty reliably transparent at 96kbit/s, whereas LAME (which really squeezes mp3 for all it's got) needs ~130kbit/s

I think encode costs are higher (although I'm struggling to find any actual data) but in my experience this is totally negligible in practice.

Of course, don't go converting all your mp3s to Opus, but if you're running a music streaming server or whatever, Opus is pretty sweet... and probably close to "as good as it gets" until/unless we figure out some crazy new tricks.