Your reliance on the 1916 treaty ignores how that deal was actually reached. The treaty was finalized only after US Secretary Lansing issued a "plain spoken threat"[1] to seize the islands if Denmark refused to sell. Thus the agreement is not proof sovereignty is inviolable, but rather a precedent that the United States is willing to use coercion when it perceives an existential threat. Same as the US feared Germany then, fears of Russia and China today could lead the current administration to again pull a page out of Lansing’s book and use similar pressure to secure Greenland
[1]https://www.execfunctions.org/p/the-danish-west-indies-prece...