From this source (called Astral-Codex, written by Scott Alexander), which is a very long read but worth the effort, it looks as if stricter lockdowns probably would have been helpful early on if people had known they would work. It also looks as if continuing blunt lockdowns probably do not pass a reasonable cost-benefit test. Testing, tracing, and voluntary decisions look very good by comparison. Here's the summary, but be sure to look at the comments as well!
1: Various policies lumped together as “lockdowns” probably significantly decreased R. Full-blown stay-at home orders were less important than targeted policies like school closures and banning large gatherings. Talking about which ones were “good” or “bad” is an oversimplification compared to the more useful questions of when countries should have started vs. stopped each to be on some kind of Pareto frontier of lives saved vs. cost.
2: If Sweden had a stronger lockdown more like those of other European countries, it probably could have reduced its death rate by 50-80%, saving 2,500+ lives.
3: On a very naive comparison, US states with stricter lockdowns had about 20% lower death rates than states with weaker ones, and about 0.6% more GDP decline. There are high error bars on both those estimates.
4: Judging lockdowns by traditional measures of economic significance, moving from a US red-state level of lockdown to a US blue-state level of lockdown is in the range normally associated with interventions that are debatably cost-effective/utility-positive, with error bars including “obviously good” and “pretty bad”. It’s harder to estimate for Sweden, but plausibly for them to move to a more European-typical level of lockdown in the early phase of the pandemic would have very much cleared the bar and been unambiguously cost effective/utility-positive.
5: It’s harder to justify strict lockdowns in terms of the non-economic suffering produced. Even assumptions skewed to be maximally pro-strict-lockdown, eg where strict lockdowns would have prevented every single coronavirus case, suggest that it would have taken dozens of months of somewhat stricter lockdown to save one month of healthy life. This might still be justifiable if present strict lockdowns now prevented future strict lockdowns (mandated or voluntary), which might be true in Europe but doesn’t seem as true in the US.
6: Plausibly, really fast and well-targeted lockdowns could have been better along every dimension than either strong-lockdown areas’ strong lockdowns or weak-lockdown areas’ weak lockdowns. We should celebrate the countries that successfully pulled this off, and support the people trying to figure out how to make this easier to pull off next time.
7: All of this is very speculative and affected by a lot of factors, and the error bars are very wide.