You can have growing deficits without actually "selling the nation" when you account for growth and inflation.
Say foreigners own 50% of US assets. When someone creates in the US a company ex nihilo and sells 10% of it to foreign investors, even though foreigners bought more US assets, this new wealth creation causes the % of foreign-owned US assets to diminish.
When foreign investors own US bonds, as inflation increases and the money supply increases, they end up owning less in real terms.
This doesn't account for investments that are limited in quantity such as real estate.
Also just because those are opposing forces, doesn't mean that foreign ownership wouldn't increase. As a matter of fact, when looking at the foreign ownership of US stocks, it has been steadily increasing.