If 20% of the population has sufficient exposure to the virus to develop antibodies, and assuming that developing antibodies will give them immunity for at least a while, while that is way short of the percentage needed for herd immunity (infection rate of 1), it should still reduce the infection rate by some amount, however small. (If I went out and infected 5 other people at the start of the pandemic, all other things being equal, under conditions of 20% immunity, now I’m only going to infect 4, because one of them (on average) will be immune.) Slowing the infection rate is a good thing, because it reduces the stress on the hospital system to have people showing up at the hospital at a slower rate, even if the final total number of cases is the same.
Does slowing the infection rate at bit compensate for the widespread presence of undetected infections just waiting to show up in a nursing home or hit a superspreading event and blow up? Of course not.