That is worse than Spanish Flu.
People love comparing any new infection with Spanish flu, but most of those comparisons miss something important. In 1920 most people worked almost to the point of death. They were typically fit enough to work until some point where they went downhill rapidly and were gone. Most people didn't spend years in a weak and vulnerable state, perhaps in care home, at the end of their lives. Now, retirement and care homes are a huge industry. We don't need something as powerful as Spanish flu to rip through a substantial section of the population. Any half assed infection novel enough to become widespread is a huge risk to the elderly, and new infections are popping up all the time. Just a couple of days ago I read about a new form of swine flu, while we are still in the middle of the COVID-19 problem. The inevitability of new pathogens needs to be factored into how society functions. Remember that Beijing was a large city when the largest cites in Europe were much smaller, and limited by their infection related death rates. Beijing was big through a single innovation in Chinese culture - hygiene. They didn't clean up the place because of an understanding of biology. It was mostly a cultural thing, because the city folk had to be more refined than the plebs in the countryside, but it worked. Civil engineering has done more to make the modern world healthy than any medical care. We need to up our hygiene game once again.
I agree, we can't compare COVID-19 to the Spanish flu, because the health of the human population and level of medical care aren't equal.
The Spanish flu disproportionately affected younger people, whislt COVID-19 tends to kill the old, as most viruses do. This could be because conditions favoured more spread in sicker, young patients. Normally when people get flu, the sicker they become, the more likely they'll stay in and not go to work, thus transmitting it to fewer people, than those who have a milder illness, which tends to make it less deadly, over time. In the case of the Spanish flu, there are large outbreaks on the battlefield and the sickest people were transported to hospital, allowing the infection to be transmitted to medical personnel, who spreaded it further. As with COVID-19, lots of the damage is caused by the immune response, so younger people with strong immune systems were disproportionately affected. Unfortunately we don't know why the same isn't the case with COVID-19.
We now have better treatments and people are more well fed, than back in 1918, which would have made the Spanish flu more deadly, than if the same disease would occur today. One of the reasons why the Asian flu pandemic of 1958 wasn't as bad, was probably due to better diets, healthcare and a vaccine being developed.
Unfortunately if COVID-19 mutates, I suspect it will be to increase the length of the asymptomatic period, rathar than becomming less deadly, because we're quarantining people.