Here’s the thing about third parties: sometimes they get elected. In first-past-the-post duopolies it’s uncommon, but it happens.
Recently I wrote that voting for the lesser evil doesn’t work.
Most of the time, neither does voting for third parties. But sometimes it does. The NDP (Canada’s most left wing party) had never formed a government in Alberta, then suddenly in 2015 they defied all the polling and won. For most of the 19th century Britain alternated between Liberals and Conservatives, then suddenly in 1924, Labour won—and this is back when Labour actually was fairly radical. The Liberal still exist (as the Liberal-Democrats), but they haven’t formed a government since.
There come times when people are upset with the status quo and truly want to change it. FDR is one, Reagan is another. In both those cases, the change was channeled thru an existing party.