Sanders is one of the most popular politicians in the US, and his political analysis and messaging remain as relevant and compelling as ever. But while his Tour to Fight Oligarchy is inspiring and important, the broad left badly needs a political vision that goes beyond Sanders.
Imo being left wing should at the absolute bare minimum require supporting the abolition of private property and ownership. Unions are fundamentally a compromise between labor and capital, therefore supporting unions is more centrist. An example of a left wing position would be supporting revolutionary workers syndicates.