I don't think that's necessary, he stands for an ultra-progressive wing with views that mainstream democrats don't espouse. He was the first one to run on raising minimum wages, paid leave, free college and single payer healthcare system before Democrats latched onto his views when he became popular.
I don't think he could have done it as an insider. Now there is a real progressive movement within the Democrat circles with longstanding dinosaurs getting primaried by more progressive candidates who stand for the same views so may be down the road Bernie might as well register as a Dem if that movement gains momentum and changes the complexion of the party.