All that is true, but it misses out a key point: Lepton is risk-free.
I could re-encode all of my JPEG photos with a better codec, but the problem is that then they have gone through a lossy compression scheme twice: once with JPEG, and once with the new codec. This could ruin the image quality on some photos, giving them nasty artefacts.
The newer codecs might even be hindered further by the fact that they are being asked to compress an image that's been JPEG'd. I don't think any of the codecs you mentioned are tuned to work their best with image data that already has been butchered by JPEG. They expect to be given the 'pure' original image. This may well cause the codec to perform worse than expected.
Using Lepton gives none of these risks, since jpeglepton is lossless. I could throw away all the JPEGs afterwards, safe in the knowledge that if I want to go back, I can.
Also, it's not really true that JPEG has been left to rot. There are lots of programs available that greatly improve upon the basic JPEG compression, while still outputting a JPEG file that any compliant decoder can read. And there's an even simpler way to shrink your photo file sizes - just drop the JPEG quality level slightly. Most cameras and programs default to a very high quality value. Dropping the default by even a tiny amount can produce remarkably smaller files, and you'll probably never notice the miniscule image quality loss.
I suspect that JPEG is so popular because it is 'good enough'. Most people and programs don't care so much about squeezing out smaller files, so they'll keep using JPEG regardless.