Mitch McConnell was majority leader on January 3rd, and he held on to it by stalling, until Jan20, 2021.No you didn't...so who is majority leader when the new session convened Jaunary 3rd, 2021. Someone has to be the leader of the Senate majority at that time.
By his stalling, he was able to put off the impeachment from the house, until after Trump left office. If Schumer had been given the reigns immediately after Warnock and Oscoff won, then the senate would have had the impeachment trial, when it was sent to the Senate, before trump left.