So consider a hypothetical case where the ultralight in RENURPP's post does indeed turn base close behind a medium category wake turbulence aircraft and the crew of the medium wake turbulence aircraft does not include 'caution, wake turbulence' with the ever-increasing R/T blurb that is required these days. The ultralight loses control on late final and crashes.
Would the crew of the medium wake turbulence category aircraft be found to be at fault for not warning the pilot of the ultralight?
No.
If it becomes the norm or a requirement to caution people then that may well change.