Bernie “can never win” because he’s not supposed to. He is part of the establishment and simply serves as a stand-in for their preferred candidate. It existed in 2016 to make Hillary look more moderate and in 2020 to make Biden look less like a puppet.
Say what you will about the RNC, but at least they have enough sense to pick the candidate their base wants. Compare that to the DNC, where Biden and Harris were the two most unpopular candidates until the primary.
All democratic governments (yes, a republic is a form of democracy) will always be a de facto two-party system. A system that has a large (but relatively smaller) third party will only see that third party being used as a scapegoat by the two largest parties. A multi-party system will simply see coalitions form that tend to be pretty much the same over time.
Look at the UK Parliament. They have 7 different parties in the current Parliament, but in practice only two. The Conservative Party has the majority. Labor is the second and main opposition party. The Liberal Democrats are third, but the only real difference between them and Labor is that they don’t openly fly the communist flag. The Democratic Unionist Party, the Ulster Unionist Party, the Green Party and Plaid Cymru collectively hold 9 seats in Parliament. They will never be able to pass any laws of their own. Plaid Cymru pays lip service to Welsh independence, but will just have to hang on to Labor to survive. The Greens are just Labor with a green flag instead of a red one. The Ulster and Democratic Unionists are basically Northern Irish Conservatives
English 
Español de Venezuela 