I just don't think that's true. Every infectious disease expert I've seen has been very confident that survivors gain long-lasting immunity, based on their knowledge of how other coronaviruses and diseases in general work. We don't have a ton of specific data about this specific virus, but it's not accurate to say we know nothing at all.

Even if it were true, there are decisions about strategy and response that must be made now and can't wait for data to come in, and those decisions are being made under the presumption that survivors become immune.