It's 2024, and it's unclear what will happen in Vermont-besides that someone socially liberal will win. Sanders has filed paperwork and has been raising and spending a lot of money already for some reason, but hasn't said anything (though he did wait until May in 2018 I believe) and is getting pretty old. A lot of prominent Democrats have urged him to run despite this.
I personally do to think Sanders will likely run again-I doubt someone would raise and spend 30 million to just not run. On the chance he decides to go away and retire, this should be safe Dem unless a socially liberal, fiscally conservative Republican gets nominated.
That doesn't fly at the federal level in New England. People like Scott, and Charlie Baker one state south, are only electable at levels where there's no risk of them being a vote for Mitch McConnell or John Thune or whomever as Majority Leader. If the Senate today worked the way it did fifty years ago or even twenty years ago, Scott would be in Leahy's old seat already and Welch would be either in the House still or retired.