The concern is that America would be too powerful if they had this power over Catholicism as well. There's no concern about waiting until it's time to appoint the next one.
At one point they took the roof off of a building because they were taking too long. Another, they changed their diet to bread and water. In one instance they stopped their pay until they decided.
You should look into the history of choosing a pope, it’s wild.
They had to. They are locked there, isolated, until they elect. That is how catholic pope elections work. Their job is to elect and then move on their normal duties and interactions.
Then they appoint somebody else and wait until he dies and then appoint Pope Americus the First. The hypothesis is that they didn't want an American pope until America is in decline and that America is in decline due to the re-election of Donald Trump. I will admit the part about Donald Trump is something i assumed with no basis and it is possible that the OP did not mean to say that America is in decline solely as a result of this, but whenever i read about America losing its international influence it's always somebody complaining about the tariffs or insulting Canada or whatever.
Anyways, nothing can end America on such a short timescale. Even if Donald Trump's recent decisions will cause the downfall of America's global pseudo-empire we are not anywhere near a point of no return and he could give up on playing "5D chess" and fix this all within a month; some opportunities would be lost which leads to some unrecoverable economic damage but we'd still be largely in the same position as we were six months ago; consequentially, any fears they may have had about appointing an American pope during a period of global American hegemony are still valid.
I don't think they had much control over when Francis died.