Looking at the worldometer COVID page I see Sweden at 23rd in the world based on deaths/1M population. The world average is 320/1M. Sweden has 1257/1M as of right now. So, yes their strategy was a partial success but they aren’t some shining example. There are tons of countries that had lockdowns, mask mandates and social distancing below them on the list.
Dr Tegnell said "If we were to encounter the same disease again, knowing exactly what we know about it today, I think we would settle on doing something in between what Sweden did and what the rest of the world has done." according to BBC.
As for immunity after infection we know people are getting reinfected we don’t know how long immunity lasts. It will almost certainly not last decades.
You are unlikely to convince me that a reasonable leader would have looked at this problem in January of 2020 and said “let it rip! Hide the old people. Hide the 56M Americans that have preexisting conditions.” We didn’t know if a vaccine would be effective, we didn’t know about long term health effects, we didn’t know about reinfection, we didn’t know what the mutation rate would be. Etc etc. I will point out that in the long run, you would expect the virus to mutate to a less lethal but more contagious variant. But before that point major lethal mutations are also possible. So infecting hundreds of millions of people and thereby encouraging mutation, seems irresponsible.