Do you sometimes go outdoors to an area that has poor connectivity?

Not all of the modern world has the same kind of network speed.

If all pages would always load in less than 0.1 seconds for every web user, I would agree with you. But this is not the case, and I doubt it ever will be considering that web pages seem to get heavier faster than how quickly the network is getting faster for the median web user.

> i dont think i have ever noticed this in practise in modern times unless artificially rate limiting my connection.

I see a lot of loading even when I'm using a pretty fast broadband (fast.com says that my ISP is 350 Mbps down and 450 Mbps up). The network speed is a function of your ISP and the target host's ISP, and you can't control the latter. If you haven't actually noticed anything, you may well have avoided websites that are slow from your ISP, either by a luck or unconsciously.

Anyway, srcset is the very obvious evidence that progressive decoding is actually needed but hindered by the lack of image format features or browser supports so it settled on a compromise.

here is FUIF (one of JXL parents) author writing about benefits of progressive: https://github.com/cloudinary/fuif In short, unlike progressive JPEG, you can serve truncated image files to get lower resolutions