Leninism, Anarcho-primitivism and Social democracy (for example) are not different approaches to “leftism” that ultimately want the same things; they are completely separate ideologies that naturally come into conflict.

In a fascist dictatorship, they have a lot more in common than opposition. What's more, there's ample room for compromise when members of these caucuses are able to communicate and collaborate freely.

The biggest hurdle to Left Unity I consistently see is Liberal Wreckers stepping in to insist any one ascendant philosophy is unserious and counterproductive, right before they form a coalition with corporationists and fascists.

you should give up the idea that all “leftists” are somehow natural allies

There's a material basis of alliance that stems from the communities that form the base of each faction.

The idea that a Social Democrat like Lulu or Sheinbaum can't form coalition with Anarcho-Prim native people in the rural Brazilian/Southern Mexican territories is demonstrably untrue.

The idea that a Leninist like Castro or Mandela couldn't lead a popular Socialist revolution in Cuba or South Africa is demonstrably untrue.

The idea that Bookchin-style Eco-socialists can't find allies in Xi's China or among the Maoist factions of North India is demonstrably untrue.

It takes work and it takes the right historical moment, but not everything has to end like the Spanish Civil War. Left Alliance isn't some impossible dream.