If we don't develop an effective vaccine then given immunity seems to be temporary (as with other coronaviruses) most of us will catch this every year so a really high proportion of us will die of covid eventually (where eventually is in many years).
By "near 100% chance" here do you mean the fraction of the population getting the disease, or the likelihood of getting the disease even if you take the appropriate precautions? Or to put it another way: imagine 1% of the population taking all the ideal precautions for the next year (and only going outside fro obvious necessities, like food/groceries/etc.). Will they also have a near 100% chance of getting it in your model? Or does 100% merely refer to # of infections / population?
London is a perfect example of how quickly this thing can spread in a modern city and how many people get sick at once. The only reason the death rate wasn't way higher is that many many people self isolated for their own protection in early March.
I know less about New York but it sounds similar.
Then they looked at a new model and realised that if they achieved herd immunity, the predicted number of people who would die, this summer, was going to be massive.
They also realised that some of those deaths would be due to the healthcare system being unable to treat a lot of people, making the number who would die even larger, and avoidably so. Presumably hospitals turning critical patients away would also be socially & politically disastrous. This started the "flatten the curve" strategy.
So they rapidly did a U-turn towards progressively increasing lockdowns, and now decreasing restrictions.
Now with a bit more thinking behind us, we also realise herd immunity might not even work as predicted, because it's not clear what are the long term effects of the virus, including immunity and ability to be infectious to others after new exposure despite having had the virus before.
Like in the USA, people aren't complying with much consistency, and it looks obvious that many people, mostly younger, don't care to protect others. The emerging anecdotes and data about long term damage when surviving do not seem to bother them either.
Nonetheless, it looks like the strategy has made a large difference to the outcome compared with what early models predicted if there were no restrictions.
And now with the emerging data about long term harm (including evidence suggesting asymptomatic carriers who don't know they had it but may have sustained organ damage affecting them in future), we can be confident the strategy, by reducing the numbers of people who get it, will have made a difference to more people than previously thought.
This doesn't stop a few people from saying that the lockdown was unnecessary because there were only X deaths and the temporary new hospitals were not much used, when X is because of lockdown (and is larger than the models predicted). There are people with no brain apparently.