That's pretty typical of zoonotic diseases, or diseases that spread from one species to another. Most of the time they're just ineffective and can't survive or are mopped up easily by our automatic immune system without us noticing. But if they succeed they're often the most dangerous diseases that can afflict us because we're not in equilibrium with them.
There was a great essay in Aeon a while back, Coincidental Killers, that focused on deadly bacteria that mostly just aren't interested in humans and evolved properties that ended up being deadly in humans by accident. Sometimes people go swimming in the wrong spot, get bacterial water up into their nose where the olfactory bulb comes in, and the bacteria discover that they can reproduce feeding on neurons and without treatment the victim dies. This is bad for the bacterium, when the person dies all the bacteria of that lineage die out too. But they weren't evolved to infect humans, they were evolved to live in their watery environment and it was just accident that they could live in and feed on a human brain too.
Very deadly diseases that can actually be transmitted from human to human are also in a similar position. If they burn too fast eventually all the humans they can reach will be either dead or immune and then what do they do? Pathogens that spend a long time with humans tend to evolve to either be less virulent or to at least kill their hosts more slowly like AIDS or leprosy. This isn't to say that diseases want to be perfectly safe if they're good at spreading, see the flu, but selective pressure in the steady state is that they shouldn't be too deadly.
When a new strain of the flu broke out in 1918 it killed 10s of millions of people. While it was still exploding in numbers there wasn't much pressure on it to be less deadly. But when it had infected as many as it was going to in the initial two-part outbreak (interrupted by the summer) the strains that became most prevalent going forward were much less deadly. The descendants of that great pandemic are preying on us still as H1N1 flu but it normally isn't any deadlier than any other flu pandemic. Sometimes there's some mutation that makes it both more virulent and more transmissible and we have a worse flu year but in general we've found a good equilibrium. But when H1N1 jumped to pigs for a while and then jumped back to humans as the swine flu public health officials were understandably a bit freaked out.
What sorts of things cause less virulence? Well, in 8 patients in Singapore we saw a mutation in SARS-2 that seem to have made the disease less dangerous but also better at hiding from the human immune system for longer. And something similar happened with the some parts original SARS too. Right now the virus is spreading rapidly and any strain that spreads less rapidly to live longer won't be anything but a tiny part of the current pandemic. But as in 1918 after the pandemic finishes, however that happens, we should expect that the surviving strains of the disease will tend to be less deadly.
And I think this pandemic will end. There have been reports of people getting re-infected with Covid-19 but no more than you'd expect form imperfect testing. We even already have a couple of coronavirus vaccines. It's been too expensive to develop a vaccine to human standards for something that causes a small fraction of common colds but immunologists were able to throw together a couple of vaccines for the common strains of coronavirus that affect dogs and cats respectively. The dog one works great but immunity fades out over a year or two just like it does from a natural coronavirus infection so to retain immunity a dog needs periodic boosters. The vaccine for cats sort of worked but not great. That, I suppose, is an indication of why it takes so much effort to create a vaccine that works reliably in humans with no side effects. There are indications that immunity to the original SARS and also MERS last longer and it would be nice if the SARS Covid 2 vaccine also lasted many years but I think having to get a yearly booster with your vaccine to this year's flu.
We'll vaccinate the rich world eventually, hopefully before we all get it. But I expect it'll remain endemic for a long time. But it should also decrease in danger over time as well. Maybe not in one year or two but it will happen if it sticks around. In 2030 what we'll have to worry about is that some new disease will cross over from a different species. A new disease that hasn't had a chance to reach an equilibrium with us yet.