You seem to be focused on a scenario where we allow the virus to spread among all groups, but that was not GP's suggestion. GP's point is that we can pinpoint who is at high risk and who is not, but that was not the case for the Spanish flu. (You talk of age as if it's the only risk factor, but it's clearly not.)

Hospitals would only become overwhelmed if the virus were to spread rapidly among high risk groups. Not many low risk individuals will wind up needing hospitalization.

Lastly, it would be more constructive to the debate to link to data instead of asserting that data exist somewhere out there which support your point.