Russell wasn't driving for Mercedes in 2020 or 2021, so nobody knows if Russell could have 1 WDC or 2 WDC, which would also mean Hamilton would "only" be on 6 WDC (which is still excellent!).
All both Mercedes drivers -- Russell and Hamilton -- can do is push to the maximum. Obviously despite Lewis being certain that Mercedes GP "never makes mistakes", he lost faith in the team while also Wolff did not offer a sufficient contract. A shame for the three-star brand, but it happens I guess -- Vettel similarly left Red Bull Racing for the prancing horse despite success, while Alonso left Renault for McLaren-Mercedes despite success...
Indeed Hamilton himself left McLaren-Mercedes despite success and could otherwise have stuck with McLaren-Honda, McLaren-Renault and McLaren-Mercedes to the present day!