I've played around with generative AI, and my gut feeling says you're wrong...
I've tried varies language models, none of them could write an internally consistent engaging story, let alone a scenario for a VN where choices impact branches. LLM does not seem to understand what it's writing.
I've played with code generation, and if the VN is a run-of-the-mill choices, branches and text, I think this is indeed doable, it's relatively consistent and simple code across different VNs.
I've played with image generation, and the amount of prompt correcting and 'engineering' you have to do to get what you want is just stupid, I admit I am inexperienced, and a lot of people can do this better, but you also need consistent characters and locations, consistent anatomy, consistent sizes over the entirety of the VN images, and that's just not feasible right now with the off-the-shelf tools.
And then just combining these? Well maybe if you're ok with low quality. And I'm also not doubting that it's possible to generate a 'decent' base of a game, just... it's going to need a lot of extra work on top of the one prompt. I do not believe we'll make enough progress to make that possible in the next five years either. Sure there will be improvements, sure we'll be able to AI generate faster and more. But we won't get to a point that it's indistinguishable from a human produced work within 5 years. I hope I'm wrong, I hope I'm impressed in 5 years about what I can do with a single prompt... For now, put me on the disbelievers list.