That's not true. Death rates are now much lower, but that seems to be mostly due to people having (partial) immunity due to vaccination and/or previous infection, and better treatment. Omicron variants are less intrinsically severe than delta, but delta was more intrinsically severe than the earlier variants, so it's not clear omicron's actually any lower than the original strain, alpha, etc.
For example, according to "Challenges in Inferring Intrinsic Severity of the SARS-CoV-2 Omicron Variant" (https://www.nejm.org/doi/full/10.1056/NEJMp2119682?query=fea...): "This meaningful but fairly small difference [vs Delta] implies that omicron, alpha, and wild-type SARS-CoV-2 have similar intrinsic severity."
The argument does not hold water. As Omikron replaced Delta, case mortality reduced immediately, despite skyrocketing cases. Population immunity does not explain this. The figure of 75% is only arrived at by correcting for underascertainment, which is an arbitrary estimate.
For example, according to "Challenges in Inferring Intrinsic Severity of the SARS-CoV-2 Omicron Variant" (https://www.nejm.org/doi/full/10.1056/NEJMp2119682?query=fea...): "This meaningful but fairly small difference [vs Delta] implies that omicron, alpha, and wild-type SARS-CoV-2 have similar intrinsic severity."