And saying Valve is morally earning money while the competition does not, its not a fair thing to say.

This was arguing in the hypothetical that Valve stopped acting immoraly. I'm not trying to argue that Valve is in the right here. I'm arguing that they are a player in this game as well, along with their competition, and so shouldn't be singled out as the ones required to change or to enforce new laws.

Valve could give gamblers 2 weeks to take their skins off the sites and then block API access to these casinos

This just gives the casinos warning so they can pull the rug more cleanly, and have more time to spin up a replacement casino.

or just shut down the API completely.

Then this punishes every other user for the actions of a tiny, tiny minority. Even ignoring users using the open market for legitimate and fair buisness, said market provides a way to obtain skins without relying on Valve to set prices or distribute skins. As such, unless Valve also removes their own lootboxes at the same time, it means that users can only interact with skins through gambling.

The one solution that would address all of this at once and wouldn't substantially affect unrelated users would be governments implementing laws against unregulated gambling. Unlike Valve, they can address the whole industry at once, and aren't punished for trying to enforce said laws.