The system gets legitimacy from the 150 million people who vote. A tiny minority of leftists refusing to participate does not delegitimize the system one iota. As a cis hetero white man, honestly my vote either way doesn't affect me all that much, other than maybe being thrown in jail for leftist organizing. But it also doesn't really affect the status quo either. I vote to benefit the people who would suffer under one choice more than the other: people of color, women, the LGBTQ community, immigrants, etc.
And yes, obviously we need to build an alternative where no one suffers, but that takes time. Personally, I prefer to at least attempt to minimize the suffering of others during that time. Accelerationism is sociopathy.