It's not a "worse case scenario", it's successful containment through quite strict quarantine measures. Since neither USA nor Italy are taking measures remotely comparable to what Hubei or Henan did when they had a similar amount of spread, we should expect these countries to have a much larger spread than Hubei did, it's really not reasonable to assume that they would be able to limit this to "around 0.1% of their population were diagnosed with covid" - it's plausible that more than half of the population could get it, and leaders of many western countries have acknowledged that.