The middle ground offers the least amount of danger, overall.
Total lockdown has to be temporary. It is analogous to repairing a car, you have to turn the engine off. But to drive the car, you have to have a running motor.
Too rapid or even total exposure would overwhelm our health systems, fill hospitals and quickly expose how (in the UK) Tory governments ignored the demands of the health and other social systems in their 10-year austerity quest: now ridiculous as they have to spend so much more to protect the economy from a single viral contagion.
The middle ground is to allow some return to productivity. Those who are low risk can keep the economy working while investing in protective measures for those who are at risk. The gradual spread of immunity is the natural way to protect us all from the virus. But our system needs revisions if we are to survive with an increased quality of life.