In reply to Thrudge:
> It seems logical to me - and this is probably where I'm going wrong - that this means the entire population of the planet will become infected, ...
Yes, pretty much, in the same way that nearly everyone has had colds and flu at some point. Though, once lots of people have had it, and are immune, it then makes it harder for the virus to spread (because there are far fewer non-immune people to infect).
What then happens is that the virus mutates, so can re-infect people who have had it before (which is why we still get colds and flu, even if we've had them before). But then the previous exposure usually gives partial immunity and means it's much milder than the first time.
Of course an effective vaccine could change the above and mean that most people don't get it -- but then, virus mutations mean that vaccines are usually not totally effective (cf colds & flu), but a partially effective vaccine can give you some immunity, so you still get it but more mildly.
> ... and this in turn will mean a death toll in the 10s or 100s of millions.
Yes, over time, and worldwide, deaths could well be tens of millions. But then realise that, worldwide, several tens of millions will die every year anyhow of everything else.
Flu kills half a million a year, worldwide. The long-term future is likely that, once we've all built up some degree of herd immunity, covid19 might settle down into something similar.
Post edited at 08:12