Coming up with a genuinely interesting gameplay loop with increasing difficulty levels and progressively revealed gameplay mechanics is a fascinating and extremely difficult challenge, no matter how much AI you throw at the problem.
If LLMs (or other "AI" or even AI tools) are able to exactly replicate the behavior of a program (game or otherwise) without access to its source code, that's technologically cool. However, that means it's possible to cheaply replicate immense amounts of human work in a way the law does not cover.
If you take a game and use LLMs to reimplement both its assets and code from scratch but players have the same movement, weapons do the same damage, have the same spread and projectile speed, and so on, then the "new" game is not really new, it's based on other people's work. And nobody should be allowed to profit from other people's work without their consent and without compensating them.
Obviously, work is hard to quantify but that doesn't mean we should give up.
2) We can also look at it from a more utilitarian perspective. When something starts as closed source, people who made it got paid already and the owners (who often did not perform any useful work except putting in money) keep making money from then on. Reimplementing it as open source does not harm the original devs but allows more people to access it and it also often leads to a much more open and pro-social implementation without dark patterns. And the paid version often still has an advantage due to existing awareness, marketing and network effects.
OTOH when something starts as free/open under conditions such as anyone building on top of it has to release under the same conditions, then a company taking that work is violating explicitly stated wishes, is making money which doesn't reach the original devs and does not promote the original work. And it also has the aforementioned advantages. When the closed version eclipses the open one, the owners are free to add dark patterns and otherwise exploit their position further.
This way open work is a global social good, closed work is only good for those who own it.
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I prefer argument 1 because it doesn't require the presence of exploitative power structures.
Either way, we should recognize there are multiple dimensions to compensation - here recognition and money. And work should be rewarded along both axes transitively.