This question, imo, speaks exactly to why I am actually an advocate for GMPCs, as well as giving GMs more tools and leeway to act in the world beyond just the rote demands required to run the game.
And that in turn all has roots in how Ive come to write villains in a sandbox environment; they're written such that they will accomplish their own story regardless of how deeply involved the PCs become in it, including not at all. Call it the Thanos principle.
Infinity War was Thanos' movie. He was the protagonist and regardless of what his antagonists did, he had a goal and the agency to pursue it. So whether he faced no resistance at all, or had to claw victory out of far more competant heroes than he actually ended up facing, or if he even lost in the end, his story was still worthwhile.
This, I think, is what made Thanos such a compelling villain (and why Loki and Zemo are the only villains thus far with any staying power, and why Killmonger dying was a horrible decision).
So, circling back to RPG land, you want to write villains like this, and while one can (and should) try to prep the different events of their story ahead of time, so that you'll always have the pieces you need ready to go, the only way to really nail down the Thanos principle is to give them agency; they need to be played, because otherwise they will struggle to come off as authentically reacting to the players moves against them. Hence, treating them as a GMPC is a good idea.
But, this isn't limited to villains. Allies to the players can be written this way too, but those have other considerations as they also have to both be likeable and reinforce the player's own agency in interacting with them.
This can be seen pretty easily in the tendency for random shopkeeps to end up being the more likeable ally characters compared to the more, problematic GMPC that tags along with the party. Shopkeeps by their nature, as usually GMs will make them some kind of likeable kooky, fit the likeability requirement, and their role in the game naturally serves to reinforce the players agency. Shopkeeps give them their choice of ways to further interact with and change the gameworld, and the agency the Shopkeep retains (ie, pay me or GTFO) all feel natural and not undeserved.
So that becomes a useful companion to the Thanos Principle, the Shopkeep Principle. Regardless of the specific role of the GMPC, they need follow that principle in order to be accepted by the players.
Interestingly, Honor Among Thieves actually violated the hell out of that principle with the Paladin character, but thats only because in the context of the movie, the character has to appeal to the audience, not the assumed players behind the other characters. Its a lot easier to use that character like he was (as an overpowered near deus ex machina like device to get them through the Dungeon set piece) when the audience neither has nor expects any agency within the story being told.
But even so, thats not to say if Zenk was written to be as he should have been if the movie was truer to what the same story would be like on a tabletop, that he still couldn't have served a similar role in the film. Theres no shortage of secondary characters in film and television that keep to the principle while also still being really entertaining in their own right, and the excellent portrayal by Regè-Jean Page could have easily been leveraged for this.
Frankly all that really needed to be changed in the sequences he was involved in was to let the rest of the group be more consistently useful. The only real mistake was having them basically be in the background most of the time he was on screen and wasn't engaging them directly.