There are two sides of this coin, that are sort of mutually exclusive:
1) On one hand, it's sort of a "competition" to gain "herd immunity". Of course if this is achievable, every country would want this, but not while disrupting civil society. So the measures makes sense for controllable scenarios, if introduction is managable.
2) COVID-19 is a completely new ("Novela") virus. We don't know long-term health impacts or long-term branches of mutation. Introduction to the masses is not without risks and costs. So if possible, stopping the virus would be preferable. There are uncountable potential viruses that are just waiting to be introduced as well, so an "open policy" doesn't necessarily make any sense. This shows how interconnected humans are today, that the "Me Only"-mantra is unsustainable, and that Capitalism crumbles in the face of real existential, uncertain adversity. This crisis is really minor.
UPDATE:
- https://en.wikipedia.org/wiki/Novel_virus
- https://www.newscientist.com/term/covid-19/
"The cause of the disease was soon confirmed as a new kind of coronavirus, and the infection has spread to a number of countries around the world."
There's no pre-existing immunity. All ages may require ICU and people without pre-existing conditions may get damages to their lungs.
In addition to handling known unknowns, states must also manage unknown unknowns.