Because as soon as leftist candidate wins, the currently existing democratic party has no reason to exist.

Sure they do, the right-wing of the Democratic Party can become the new, saner right-wing party. With the existing GOP being treated as the toxic waste dump it's become. It's always good to have reasonable opposition in government. They opposing sides force each other to think critically about their positions and consider them from angles that they might not have. The problem is that this needs to happen between parties which are committed to the betterment of the country and upholding democracy. Right now, we have a fractured Democratic Party which is having that push and pull happen inside the party, weakening it while the GOP has turned into a cancer which is being led by bullshit. Unfortunately, the cancer is currently too popular to effectively cut out and our political system makes the rise of a third party incredibly difficult.