Let’s start with the understanding that there are several possible futures, and that different people assess the likelihood of each future differently.
One is the possibility that regions will be able to suppress SARS-CoV-2 infection rates to very low levels long term by some combination of ongoing social distancing, intermittent lockdowns as needed, comprehensive testing and contact tracing with self-quarantines, and isolation from imported cases by whatever means required.
Another possibility is a vaccine fairly quickly developed that safely provides good enough protection for long enough periods that is available to and accepted by enough people that herd immunity is achieved safely.
And the last is that infections occur to some segments of populations over some period of time resulting in regional herd immunity, or absent full protection at least protection from severe disease upon re-exposure and potentially decreased transmissibility. Under this possibility there is the possibility of controlling the who gets infected the most (protecting the higher risk while lower risk individuals build the pool of resolveds) and the when (as a surge that overwhelms systems or is timed with influenza, or flatter spread out over non-influenza times, for example) by impacting behaviors.
Note that if neither of the first two occur, for any reason, inclusive of failure despite best efforts, the third is what occurs.
Next let’s look at what is and is not known.
Not known is what level of infections is required for herd immunity in any specific society or region. 60 to 70% as the quoted guess is based on a belief that this germ would act like an influenza. It does not. the number is an unknown.
Not known is how many in any population function as not susceptible in any specific population. Studies on blood sample stored from years before the pandemic demonstrated that about 30% of samples contained T-cells that specifically respond the SARS-CoV-2 before the germ ever existed in human populations. Non-specific antibodies from other common cold causing HCo-Vs provide some protection against SARS and MERS and are speculated to do the same against SARS-CoV-2. Children, especially those 10 and under, seem to not transmit much even when they are sick with COVID-19, which may mean they function more similarly to non-susceptibles than as susceptibles in any model.
Not known is if those who have had asymptomatic infections some sizable fraction of whom do not have specific antibodies significantly elevated are susceptible or not by way of specific T-cell responses. Not known is how long protection from infection lasts. Not known is how significant mutation of SARS-CoV-2 will be and how either the state of having resolved from natural infection or having specific antibodies by way of immunization each do in that case.
Known is that a larger share of the population being susceptible will result in a faster building and higher surge than a smaller share, whether or not full herd immunity exists.
Known is that even a moderate surge occurring at the same time as influenza hits would overwhelm systems horribly and lead to many excess deaths.
Not known is whether either of the first two possibilities will occur, can occur, or what sorts of collateral damage attempting them might incur. Just not known.