"The denominator of total cases is grossly understated and thus % is grossly overstated. Everyone knows this and so do you"
I suppose it means what you mean by 'grossly.' I believe that the total number of cases is understated. However, I also assume that professional epidemiologists, such as those who work for the World Health Organization, also understand that not all cases have been reported and they have factored that into their estimation.
I don't know how they came up with the 3.4% number, but I also believe they know what they are doing and have studied the evidence far more thoroughly than you or I have done. They understand the limitations of evidence better than you and I do. I'm reporting numbers provided by the top scientists in the field.
Although the Surgeon General has provided evidence about covid-19's mortality across different age groups, those numbers do not directly challenge the 3.4% rate. Since America is heavily weighted towards people over 65, the increased risks they experience can still lead to a population rate of 3.4%.
For the sake of argument, let us assume that the real mortality rate is only 0.2%, double that of seasonal flu. America has somewhere around 34-40 million cases of the flu each year. But the number of cases of flu is influenced by people who get the flu vaccine and are protected from the flu thereby. (I've gotten flu shots for decades and have not had the flu in more than 30 years.)
Flu shots don't only protect the person who got the shot. It also protects people who don't get infected because many around them have received flu shots.
Without any native immunity to covid-19 or a vaccine, the infection rate for covid-19 could easily be much higher than the flu. How many millions more? What if 100 million people caught covid-19? How about 150 million?
At 100 million infected and 0.2% mortality, that would be 200,000 deaths. 150 million gets us to 300,000 deaths. But we can't forget the significant number of people who require medical treatment. It's not just those 200,000 - 300,000 folks who need to be hospitalized.
Again, you may take comfort that children and young adults are largely spared. I'm not in either group but I am in a high-risk group, along with millions of other Americans. My wife is in an even higher risk group because of her primary immunodeficiency and a history of respiratory illnesses. She's got a trifecta of risk factors.
Again, 3.4% is the best estimate we have right now. It may not be very good, but one has to at least acknowledge this is the current scientific consensus, albeit based on early numbers.