As for what may happen when the U.S. Supreme Court ultimately gets this issue, note the language in the original U.S. Constitution. It established no uniform law of elections for federal office. Instead, it adopted diverse state election laws by adopting the standard of the same voters who could vote for "the most numerous branch of the state legislature." As a result, in the first U.S. election, blacks and people not owning property voted in some states. And in one state, interestingly, New Jersey, women voted.
The same adoption of varying state election laws occurred in 1913, when the 17th Amendment was passed to make Senators elected by the people, not appointed by the state legislatures. That amendment repeated the "most numerous branch" language from the original Constitution. This is one reason why the U.S. Supreme Court has accepted as constitutional that some states bar convicted felons from voting, with varying methods of lifting the prohibition. By contrast, other states allow felons to vote, no questions asked.
It is also why the Supreme Court has upheld voter ID laws. Some states require voters to show picture ID, usually a drivers license, though state ID cards for disabled people in lieu of licenses also serve. Other states have no such provision. Again, that difference in state election laws is fully constitutional according to the Supreme Court.
Defendants in this case claimed that the 1995 Thornton case controls. There the Supreme Court struck down state-established term limits for Congress as unconstitutional. Yet one of the reasons the Court so ruled was that it interfered with the voting rights of the sovereign people. In this case, upholding the rights of the people to vote as they choose supports, not opposes, the state provision.
More likely than not, when the Supreme Court rules on the recall of Senators, it will conclude that this question belongs to the sovereign people in each state, and that others may follow New Jersey’s example if they so choose to make recall as much a part of their election laws as the initial election which places the Senators in office in the first place.