If it's 40% instead of 100%, wouldn't that just mean that natural herd immunity could occur after a higher percentage of people have been infected? Like, instead of 60/70%, it's 90%?
40% of the people who get infected never develop antibodies. This means that only 60% of those who got infected developed antibodies.
Typically you need 70% of greater of a population to have antibodies to develop "herd immunity" for a disease like COVID. If only 60% of the population can only get antibodies via infection then the population will never reach the 70% figure via infections --- vaccines are the only hope.