Good points, but let’s not forget that citizenship comes with both benefits and responsibilities, both for the citizen and the state.
Being a citizen in most countries comes with certain civic duties, like military service or jury duty, and even mandatory voting in some. Likewise, the state has to hold up its end of the bargain even if the citizen costs more money than it nets (as is the case for every retiree). By turning citizenship into a fluid, subscription based concept you open the doors to both state and citizens trying to game the system. Imagine a state canceling your citizenship when it determines you’re no longer useful, or millions of citizens canceling theirs in wartime.
Of course, the article mentions that a fluid citizenship can come in varying degrees, so it’s not a simple dichotomy. Some perks and duties can be transient while others last a lifetime.