The mask use policy change was initially about the drain of N95 masks from healthcare professionals to everyone, when there was insufficient supply. Later it was discovered that while masks are not generally a good barrier to getting it, it was a substantial barrier to giving it to others and with the difficulty of knowing one was infected, widespread use was needed to catch cases of those who did not know.
Not sure about parks, but at the outset there was no information of just how contagious and how persistent Covid was, so the emphasis was on the better too much concern side.
The direct methods of investigation to determine the values of all the variables are unethical and widely frowned upon. So only via observational study is available and from that conclusions about the disease shift. In addition, changes in supply availability also can change what is the best current thing to do.
It seemed like a really bad sign when China, significantly sensitive about its appearance to the outside world, let out that it was walling off entire cities and welding shut doors on apartments. Whatever they do on human rights, they tend to keep it quiet and this event they allowed information to exit.
The US should have put everyone entering the country into 2 week quarantine on entry. Instead passengers on international flights were allowed to spread the contagion to ground crews and airport employees (who went home and took it to their friends and family before coming back to infect other plane-loads of people) and then continued on with other flights across the US. If I wanted to create a policy to spread a disease with maximum effect that would be my choice.
The present problem across the world is that the little bug can be augmented by evolution. Viri are not substantial and prone to breaking and incorporating other components, and it looks like there are already new versions that are more effective at spreading, are hitting younger people pretty hard, and for which the present vaccines may not be effective at generating a useful immune response.
Humans aren't to the Spanish Flu or Black Death situation this moment, but the more quickly and certainly measures limit the number of humans that these variations can develop in the better. It's why firefighters just keep pouring on the water long after most people think the fire is out - just one little ember in the wrong place and boom - another huge outbreak from fire or disease.
My crystal ball says this isn't over for the next decade, with odd outbreaks of citywide proportions on a regular basis through most of it. It also says at least two or three new vaccines will be needed to take on whatever develops in any of the billions of people somewhere on the planet, but the tech to create them and then produce them will get much much better.
Right now that major spot is India where a political candidate anxious to reassure people all was well lifted restrictions just before the election. It worked. People were reassured and they elected the guy. They now have a million new cases a day and burial services are outstripped and it's a perfect location for one little transcription error to maybe make things much worse. This video explains the various forces at work:
Back to ventilation - I'd like to see top down air supply with floor level full perimeter air returns so particles are carried down and generally kept in a boundary layer at the floor and limited head-level horizontal flow. Coanda can be medicine's friend. Unfortunately this means big changes to building architecture. I suspect getting this to work will require a lot of work and retrofitting is not easy. However this is applicable to all respiratory contagious diseases so even when Covid is gone, it will remain useful for a very long time.
