This is a very concise explanation.I don't know the game should be totally open-sourced just to have their assets on the blockchain.Blockchain could be a part of their environment where assets are stored.How else do you expect games to make money if everything is given out for free ? One major issues of having assets on the chain is what if user loses access to their private keys ? The game developers wouldn't be able to restore his purchases if any...
Open-source doesn't mean that the whole in-game content is free, it means that the source code is open so users would have the ability to play the game even if developers would make some changes that community is not agreeing on - this is necessary to make the whole game decentralized, because storing assets on blockchain is pretty useless if developers still control the whole game - for example they can change in-game models, stats for blockchain assets and also ban accounts. There's a good article covering this issue in CryptoKitties:
https://www.coindesk.com/scratch-cryptokitties-isnt-ethereums-vision-apps/Reversing payments is another big problem - anyone who played popular MMO's knows how often accounts get hacked to steal in-game currency and items or resell them later - and thanks to centralization original owners are usually able to get back their stolen items.