Democrats in office right now will never, never put up a fight. Ever. And neither will republicans. People can scream all they like - nearly all high-level American politicians today got into politics for their own benefit, and are happy to continue receiving "lobbying" money in exchange for the destruction of the world. Vote for them, don't vote for them, they don't care.
The lesser evil proclamations are not at all about reversing the trend, because it won't be reversed. The only thing that will reverse the trend within this generation will be outright revolution. Voting for the lesser evil is about minimizing the damage in the meantime while we organize the revolution.
If we decide to go the route of changing the democratic party slowly from within, like with Mamdani, then it will also work, but only by the time local city council and mayoral candidates advance in the government enough to become presidential candidates, which will be decades. And even in that case, we need to minimize the damage to make sure the country survives until then.
The democratic party won't ever put forth a leftist candidate until there are literally no other candidates for it to choose from, so we either wait until we've swapped everyone out, or we revolt.