There have always been unions that organized in a manner that avoids this situation. (The CNT, the IWW, the Zapatistas (1990s) and, more.) In fact, this act of holding on to power has always been one of the major Anarchist critiques of Marxism. And, Anarchists were very present in the unions of the early 1900s.
One of Capitalism's and Authoritative Communism's finest achievements has been burying the history of unions. They've done it so well that many current unions are unaware of alternative power structures.
It's unfortunate because there is so much to love about Anarchism.
Democracies are ugly, inefficient, buggy, and bloated. Most people don't get what they want, but they agree to form a union because they know that they'll get more than if they were ruled by an autocrat. Democracies are pragmatic.
However, some people naturally want to push their vision onto others. And, there are constant forces at play to do so.
I think the democracies that work best have some common culture. They seem to work better because the core differences are more tightly clustered.
Media owners will similarly run false stories, exaggerate and blow out of proportion any instances of union corruption whilst simultaneously downplaying and ignoring vastly worse instances of corruption by the owners and managers.
"Any kind of group activity requires organization. But as soon as one starts to organize one creates a division within the movement between the organized and the organizers, between ordinary members and officials. The latter quickly acquire knowledge and expertise that set them apart and give them power over ordinary members. The officials begin to derive personal satisfaction from their place in the organization and seek ways of consolidating it. They acquire an interest in the continued prosperity of the organization. For ordinary trade unionists, their union is just one interest in which they have a small stake. But for the paid officials the union is their employer. Preserving the organization becomes more important than helping it achieve its goals. As radical action may bring government repression, the apparatchiks moderate."