Sherrod Brown isn't going to do a lot of good when Democrats can't pass any legislation for four years and we lose the Presidency in 2024.
Warren, Sanders, Brown are at my very tippy top of best possible Presidents, but I'd take someone more conservative like Beto if it means we have a chance at a Senate majority.
Vermont requires an election in 90 days. And if he timed a resignation right, it could be combined with the Presidential election. I assume Welch would get elected Senator. So really nothing to worry about there.