I get what you're saying, but I strenuously disagree with your central assertion that there's no point in legitimate criticism of a sitting president as long as there isn't currently a better president available.
You don't just get a free pass to commit crimes against humanity because there's nobody lined up who wouldn't.
That's lumping the US president in with the likes of Viktor Orban, Vladimir Putin, Benyamin Netanyahu, and other presidents whose subjects aren't allowed to criticize their worst actions.