I think you are missing the point.
You would be right if we had a healthcare system that was ready to handle and treat a potential lot of COVID-19 patients; if we could isolate every vulnerable person 100% until the pandemic has crested; if we weren’t seeing cases of younger folks getting completely ravaged by this disease. Chickenpox are a tolerable discomfort; violent, viral pneumonia would be unethical to knowingly expose someone to, even with a 75% chance of being symptom-free.
You are suggesting a simple plan for a very complicated disease. You’re right that isolation probably won’t work on its own, but we need to buy time to develop a treatment or a vaccine or something that will give us some control of the situation. Our approach right now is extreme, but the alternative is ugly and unethical.