What you see as a positive I see as potential for implementation details to diverge and cause user issues. We already have issues with WebP and AVIF rendering in some versions of Safari, and those are well established standards, not a lone wolf extension.
In theory an extended hacked PNG would have perfect backward compatibility, but the complexities of codecs more likely means it's going to appear as bugs in several implementations. Best case it falls back to SDR, but it's also possible some broken implementation leads to HDR content being displayed in a SDR setup and everything getting all washed out.
Having the same file format/extension actually host several different versions never works well. Graphic artists and support teams have to deal with rendering quirks like this all the time because some developer thought their hack was a genius overload :/
