To get rid of the virus, you do not need to eliminate transmission, you need to reduce transmission so that it is slower than the recovery rate.
Eliminating large crowds, on its own, reduces the transmission rate. So does everything else, like washing hands, etc.
The chance of a person being infected is driven by how contagious the pathogen is, and by how many people already have it. This virus is moderately contagious (it’s nothing compared to, say, measles or mumps). However, left unchecked, the numbers will grow exponentially, increasing the chances of infection.
When it’s widespread, only the nuclear option will work, with the nuclear option being a lockdown, thereby eliminating direct contact between people. A virus like this coronavirus does not transmit very well via indirect contact. However, if you have 1,000,000 people with the virus, a 0.5% chance of transmission still means that many people are being infected, especially at the beginning of the lock down.
Moreover, many people decided that the lockdown wasn’t for them, and they crowded at the beach, at parties, etc, even after the NYC lockdown. There were also low paid “essential” workers who were not provided with protective clothing or gear, despite constant exposure, etc.
However, even an imperfect lockdown reduced transmission enough that recovery started catching up with transmission. That is why the infection rates slowed in almost every state,
The problem with states opening up is that the rules put in place to reduce transmission are based on assumptions of much wider adherence to these steps than will actual occur. They also ignore the fact that there are many people in every state who will not only ignore the safety regulations, but do the opposite.
Masks are a typical case. These masks do not do much to protect the wearer, they protect everybody else. Therefore, they are useless unless the vast majority of people actually wear them. Imagine that 40% of people do not wear masks, or take any steps. Now figure out that up to 70% of these people can get infected, and that about 30% of the people without masks will be have COVID 19 and be contagious. That means that about 12% of the people with whom you interact are infected and contagious, and that your mask is only 30% protective.
So 60%-70% in full lockdown and 20% in partial, while 10%-20% ignore lockdown, will reduce transmission below recovery. However, 80%-90% in partial and 20%-10% ignoring all precautions, may not, which will result in exponential growth of cases once again.