At that point he has no ongoing political motivation to preserve his phony Democrat identity.
Phony Democrat identity goes far beyond Sanders. AOC, Omar and the squad, and many others are not married to the DNC or the Democrat Party, even though they claim to be Democrats. Some are Nationalist Socialists, some are Islamic Supremacists, some are Black Nationals, some are Aztlan Mexican Nationalists, and of course LGBT Activists among other various special interests. Only a percentage of the Democrat Party actually consider themselves true Democrats over whatever cause they represent. They join the Democrat Party not because they want to, but because it is a binary choice and they cannot be Republicans.
Phony Republicans is a very well known issue. The same idea applies. Only a percentage people who call themselves Republicans actually have loyalty to the GOP. And it is for the same reason. It is a binary choice and they didn't want to be Democrats for whatever reason.
If America were more like other countries, there would be a dozen Parties or more to choose from, and several of them would ban together to form a ruling coalition. Because America is either (R) or (D), people are forced to choose one even if they do not really believe in the Party platform.