Another films: "I grow tired of asking this so it will be the last time". The other countries you cite have not hit their 15-20% threshold. Therefore you don't see it at work. Taking them each at a time:
China: Surely you don't advocate the harsh measures undertaken by China? In Hong Kong, where there are limits still on the ability of China to use its techniques, and where western press still has a presence, they are on their 3 wave. This despite a universal mask requirement, test and trace, and severe limitations. That said...they may have hit the 20% marker in Wuhan...we simply don't know due to the Chinese government's propensity to falsify information. But, few people (outside of yourself) have said a Chinese style system should be adopted here.
Japan: Universal mask usage has been unable to contain the virus and they are now in their second wave and second round of lockdowns. Mind you, I think masks helps, but the Phillipinnes also shows the limits of mask usage.
Korea: Actually shows the limits of test and trace. Despite one of the best test and trace systems, not to mention enforced quarantines, they haven't been able to contain the virus and have periodic outbreaks...like a fire you have to constantly try to smother out but are unable to.
New Zealand: Island...got lucky no superspreader event early....didn't have very many tests per capita but deployed what they had early wisely. Sealed themselves off from the rest of the world and as an island they are able to effectively do that (our southern outbreak is partially fueled by the southern border).
Finland and Norway: Have no mask requirement and the lowest levels of mask usage in the western world.
Germany is probably the most interesting case. If my lockdown theory is correct, they just timed it perfectly. They got lucky with their lack of superspreader events. Weather may be a factor. They have less testing capacity than the US and face mask usage is certainly less compliant than SoCal. Even so, it's experts in the last couple days have warned that like France, they are seeing indications of a second wave, particularly in the east which is less densely populated and wasn't hit as hard the first time round.
Spain: toughest lockdown in the western world and universal mask usage (and no stupid bandanas or cloth masks allowed like here). It's actually the best case against the 15-20%. The rebuttal is that the outbreak is currently mostly in the Catalonia area where the initial wave wasn't as severe. But if Spain accelerates again it will effectively kill the idea there's a breaker at 15-20% (which leaves only weather as a likely indicator, given the Swedish experience, but it would also explain what's happening in Germany, Finland and Norway). Either way, Spain stands as a testament that short of a perpetual lockdown, there's not much anyone can do....because if it's out there Spain has done it.
Now this is just opinion but I actually think clinging to false hope is more dangerous. It leads us to do truly stupid stuff.....like when at least 1/3 of our population has decided social distancing is over and they're not going to do it anymore and completely fill up various parks this weekend with full contact, punishing kids (and not businesses like Downtown Disney or Knotts) by taking their sports and schooling away.
