Washington Post editorial page wisely notes that the past few months’ worth of Senate appointment chaos should serve as a reminder that appointed senators are a bad idea. We ought to do special elections.
This also reminds me of the reverse problem, namely that in the event of (God forbid) a mass-casualty terrorist attack on the U.S. Capitol, the requirement that members of the House of Representatives be replaced via special election could produce a disaster. In other words, if someone managed to somehow kill almost the entire congress during a State of the Union address we wouldn’t be able to stand-up a new House to authorize the needed response measures. The only reasonable course of action would be some kind of extra-constitutional rule by presidential decree and it might be difficult or impossible to actually return to the rule of law.
Ideally, then, the ordinary rule would be to replace Senators or members of the House via special election but to have some kind of special proviso for filling seats with appointees in case of a mass-casualty event.