Strongly disagree. While a lot of work does go on to art assets which should be simpler to migrate, the code is absolutely what makes the game. There are tons of very successful games with low quality or stock assets, there are very few popular games with broken code.

Even then, it’s still a lot of effort to check every asset you’re using to ensure they work as expected in your new engine.

You’re completely right

I agree for a specific scenario: if you don’t use many unity specific packages or assets. Then, perhaps you are correct, still I don’t blame anyone staying even in that case, as it is still daunting to take on such a task.