[aninhumer]

>Are you really suggesting that anybody in the world who wants to walk into the USA should be allowed to?

Maybe block known criminals and bad actors, but otherwise yes. If your argument as to why we should exclude people is based on the idea that it would make standards of living worse for people here, that's a) probably not true and b) only makes sense if you think the wellbeing of the people here is more important the wellbeing of people elsewhere.

>It seems like that nullifies the existence of the nation itself. Why have a border?

Not really, there'd still be a government and a system of laws that apply within the border.

But beyond that, why specifically do you think it would be bad to nullify the existence of a nation?

>Why have a military to protect that border?

In as far as a military is necessary, it is to protect against other militaries, not non-violent civilians entering the country.