Seems like dems want this changed because it could work, legally.
I think the real gist of the issue is how state's determine electors. It may be there is no direct connection, constitutionally, between a vote of the citizens and the electors. Citizens could vote for state legislatures, who would, then, choose electors. Similar to how Senators were chosen, for example.
If a state can determine how electors are chosen, and they choose to honor the result of a vote of the citizens, they could also decide not to do that. Changing election laws arbitrarily, as seemed to have happened in 2020, could lead to that with no recourse to question the electors later at the fed level.
Bottom line, I think, is that Brandon was appointed, not elected, and that may have consequences, depending on how he was appointed. If it was outside of election law, and/or under influence of foreign actors, there may coup/act of war implications.