Far-left policies like "Medicare for All" and so on are incredibly popular and indeed many left policies once implemented, like social welfare and so on, stay on the books. Obamacare was quite popular and is quite popular with a lot of Republican voters, it's just that in their district and place it's not called Obamacare or ACA. That was the reason why McCain cast the famous thumbs down during the Obamacare repeal bill...because again Red voters liked the policy once it was implemented. Again, the Republicans had a trifecta in 2016-2018 -- the WH, the Senate, and the House. In that time the only major legislation they passed was the tax cut. They didn't repeal or overturn Social Security, cancel Medicaid, cancel Obamacare or anything. So I don't think a candidate going far, far left is fated to lose.
Left-wing policies are popular and have staying power, they just need to find a way to get it across.