That isn't how math works. If the denominator is infinite, then the mortality rate is zero. There are not an infinite number of humans, so the mortality rate couldn't reduce to zero. The US currently has 301 deaths with approximately 25,000 confirmed cases. That works out to a mortality rate of about 1.2%. If there are 250,000 or more actual cases, then the mortality rate is actually in line with the flu. It is extremely possible that there 250,000 or more people actually infected in the US. It is also extremely possible that there are less than 100,000 people infected. The denominator does matter because math doesn't change with fear.
If you choose to believe that the experts in the field tell us, then you have to believe that the mortality rate isn't known at this point because that is what the experts are saying. Several experts have said that they believe the mortality rate will end up in the 1% range, but they are stating rough opinion not fact.