2008: "Let's vote Obama!" - Obama becomes President
2012: "Let's vote Santorum!" - Santorum loses primary
2016: "Hillary's got this primary, vote Trump!" - Trump becomes President
2020: "Let's vote Sanders!" - Biden becomes Democratic presidential nominee (at least)
Has it never not been a failure?
It was never meant to be a winning strategy. It was meant to keep the number 2 viable long enough to force number 1 to expend resources longer than otherwise would be the case. Nothing more than that.
And, BTW, your list could not possibly be more incorrect. Proving you are speaking to an issue you personally know nothing at all about. Which is all too common on this board.