> the average domestic connection being gigabit

This strongly depends on where your domicile is, even in Europe and the US. And lot of the users live outside these areas.

> JPEG was fine in the days we had much slower connections

Except that waiting for a large picture to load was very much thing, and in many cases still is.

> What are the benefit for the final user?

A number of users are still on metered mobile plans, and have little storage on their phones. With the current situation in electronics production, new phones are not going to offer more space, cheaper; to the contrary.

Also, publishing larger / better quality images becomes easier and cheaper.

> What are the benefit for the final user?

Many that you can search the web for, but to address the specific points you mentioned off the top of my head:

- Lower cell data usage for users

- Not everyone has gigabit fiber

The issue for me is that RAW sucks, and Jpeg also sucks. Jpegs are standardized and display anywhere, but are also super lossy. RAW, like fish, is not actually a thing beyond a sorta "I know it when I see it" deal. It's non-standard even between camera models from the same manufacturer, and there are like 10 pieces of software that bother with this bullshit and afaik none of them are OSes or browsers.

First, try explaining to a non-tech person the difference between jpeg and png and they commonly scratch their head. It's nice to have a single format covering lossy and lossless, just meaning "picture". Whether it's a photo where lossless is ok or some icon where it isn't should be insubstantial.

Second, the "all connections are fast, what's the point" is one reason why the web is so slow. Connections are often slower than the dev with his 1gbps connect feels every day. The average website loads slower (in wall clock time) than it did 15 years ago. Smaller files and fewer network round trips would do everybody a favor.