All viruses evolve. RNA viruses (like Coronavirus or HIV) evolve super fast. Literally over a few days, they can evolve completely different characteristics. When a virus is introduced into a population, for technical reasons its evolution rate increases radically. So Coronavirus might evolve quickly into something very different.
There are several flaws in that. If you take the total number of cases in a pandemic that has run its course, and compare it to a pandemic that has not run its course, you are comparing patients who have either died or recovered, to patients who are at the beginning of either dying or recovering. Lets say there were 100k patients, 2k have died, and 2k have recovered. You would say that there is a fatality rate of 2%, even though a few weeks later 50k of them have died, and the more accurate rate would be 50%.
Then there is the problem of figuring out how many people have the Coronavirus. Potentially, some people have it and it is too mild to detect. But also there are people who have other diseases that are being misdiagnosed as Coronavirus. It becomes quite complex in the moment to figure it all out. Maybe a year or two from now we will have good numbers.
And if the Coronavirus becomes more lethal, like the Spanish Flu did, who cares if it previously "only" killed 2%.