You bring up a good point about Herd Immunity. It has virtually rid the world of Smallpox.
HI works if a person who is infected goes interacts with mostly immune people. They cannot spread the disease. But the percentage of immune (either from recovering from the disease or by vaccine) is pretty high.
It's estimated that Covid may reach 74%. Yet Measles and Pertussis have reached over 90% HI and are still a danger.
The Spanish Flu was almost under complete control by early 1918. The second wave was far more deadly due to complacency and a possible mutation.
I think a combination of physical distancing (We want to keep close socially, as we can electronically), vaccine development, and antiviral drugs are going to be the key. But when messages from those with ulterior motives (The luddites in Britain blaming cell service, or those more worried about their stock portfolios) or political leaders conflicts with the scientists, there is a real problem.
For those of us who are over 65, I do not see that we will be able to interact with other people for at least a year. The chances of infection are going to be high. Buying something at a store can mean receiving change that has been touched by dozens of people, the door has been touched by hundreds. And even if only 1/1000 carries the virus, one will almost certainly be exposed.