JXL has clear advantages and saying you don't see them means you either didn't attempt to see them or willfully ignore them

I'm not sure if the parent comment means to say, but I could choose to interpret their meaning to be:

PNG/JPEG have strong support (nearly everywhere). I think of them almost as the modern TIFF - relatively simple, and almost everything supports one or both of them (few to no issues).

JXL may eventually get there (that indeed seems to be its goal), but for now I wouldn't drop e.g. PNG or JPEG support in favor of JXL now.

And arguably, perhaps never drop support; its such a prevalent format, even if all software (prevalent or otherwise) supports the format in 10 years, there will be such a huge stockpile of JPEG/PNG that something will need to be used to read/convert.