Your statements about lifetime immunity from viruses is not correct.
Immunity after any infection may be in the range of virtually nonexistent to livelong. For some diseases, such as measles, it is livelong. For SARS or MERS which are similar to C-19, immunity lasts about 1-3 years, with partial immunity that tends to decline with time. It isn't known yet how much immunity there may be for most people from C-19 and/or how long it lasts and how this may vary for different people. For some common colds, it may last less than a year. For some diseases, such as TB, it is difficult to determine what is a new infection or the flare up of an old infection.
It isn't yet understood whether mild infection or asymptomatic infection builds up sufficient protection against reinfection for C-19. Add to this, all the other confounding variables such as whether people with a prior infection might differ from people who haven't been infected, prior risk factors, genetic confounders among others adds to the struggle and need for good research and time to begin to sort out these issues before coming to premature and unfounded conclusions about all these open questions and exposing the population to illness and death.
Add to these unknowns about developing immunity the concerns about immune enhancement. This is a big one. This immune system backfiring can be manifested in a process in which a virus leverages antibodies to aid infection -- kinda hitches a ride on the antibodies to infect the cells. Or, it can induce autoimmune illnesses in which the body is triggered to turn on itself and attack its own tissues as occurs among a variety of horrible incurable and lifelong diseases such as Type I diabetes, Rheumatoid Arthritis, Lupus, M.S. Hashimoto Thyroid disease and many, many others.
These are the reasons that a vaccine against dengue fever and other illnesses hasn't yet been developed. It can make the disease worse. It is known that getting dengue fever a second time often leads to far more serious and deadly illness than the first exposure which may have been either mild or asymptomatic. And right now, there may be new evidence that some people do get C-19 a few months after their recovery and with more severe symptoms.
The science of virology, microbiology, and epidemiology are very complex and well beyond the expertise of economists and traders. Decisions of great consequence are being made based on beliefs, part truths, conspiracies and wishes. While there's much that is not understood, the best proscription is to follow the advice of scientists and know what you don't know.