I'm with FoolsGil on this one. Zenkais are indeed Deus Ex Machina and sloppy writing. The Deus Ex Machina part comes in the fact that its a miraculous tool that makes the characters as strong as the writer needs them to be without any particular rhyme or reason - "an unexpected power or event saving a seemingly hopeless situation, especially as a contrived plot device in a play or novel."
To your point MasenkoHA, we know they exist ever since Vegeta explains them, but what each character gets from them is wildly inconsistent and completely up to the whims of Toriyama-san who immediately abuses it. It's inconsistent right from the start. Vegeta himself barely gets any increase after he leaves Earth, but after fighting the Ginyu's he gets a x16 boost to match Frieza's 1st form? Then Goku himself gets something of a 33x boost in a single Zenkai. The "unexpected" part of the definition applies to the sheer volume of power granted from this mechanic since it only exists to solve current problems (i.e. surviving and making it to the next stage of Freeza's boss fight). And if we retroactively apply this to the story, no previous zenkais have been even 10% of what we see on Namek. Every application of it on Namek is indeed unexpected and useful to keep the user alive in a hopeless situation, mostly vs Freeza whose power they are trying to hopelessly overcome.
On top of that, Zenkais were a mechanic created on Namek that for the most part stays on Namek. It's clearly an means to a very obvious end.
They really are a cheat code though, especially on Namek. They're a cheat code for Goku and Vegeta and they are a cheat code for Toriyama Akira. It's just a convenient means to an end, same with Namekian Fusion and the Guru powerup at the time. I just find Zenkais way worse became of just how much they were spammed on Namek, how poorly they were explained combined with how inconsistent they were, and how quickly their relevance disappeared from the story without so much as an explanation only to resurface again for a massive story beat, again, when convenient to do so ... then again decades later, when convenient.I don’t think they were ditched as much as it was understood that it could only get you so far. Goku got a lot further from actually training rather than depending on these boost like Vegeta had. That’s why it’s funny to me fans are acting like it was some huge cheat code to the story.