The Democratic party is in unacceptably poor shape with no clear path back. There is no point in being in an irrelevant party if you want to have a say in American politics.
Obviously, you know nothing about the state of American politics over the past few years. Before the election, we were all saying this about the GOP, and Trump's election didn't change the fact that Democrats still control the terms of debate in this country.
The Democrats have probably controlled the controlled the terms of the debate for the past 55-65 years because of MSM. The Democrats will probably always control the terms of the debate in this country because of MSM.