A hypothetical Covid-2 which is 20x deadlier than this pandemic and kills across all age groups would be a really good candidate to contain via contact tracing well before it reached the point where we needed mass lockdowns - assuming that Covid-19 hasn't totally destroyed trust in the scientific and medical establishments by that point. Remember, 20x deadlier means a lot more people showing up in hospitals, which means a much higher proportion of cases can be detected early on in the pandemic without needing mass testing. Covid-19's fatality rate is probably close to optimal for causing the most damage overall.