What you're suggesting probably will happen, just not right now. As people go back to work, most high-risk people will continue to stay home (hopefully with government support). But as OP points out, we don't even have a full understanding of who's high risk. Every day we lockdown we're buying time to figure it out. We're also buying time to develop medications, manufacture equipment, study immunity, ramp up testing, and so on. It can't go on forever, but these are valuable days.

Several European nations have already started to ease lockdowns. High risk people still stay home, but a lot of people get to go back to work. If the US gets it's shit together, that can happen here too.