"...claiming that the license can be revoked at any time."
Of course a game company can revoke their license at any time. The company grants you a license to use the product, and a license is not an obligation on their part to provide the product, or a right to use it on your part.
There's nothing illegal about a license or TOS that has clauses which stipulate when the license or TOS can be revoked changed or revoked.
It depends on the terms of the license.
I'm not a lawyer, but my understanding from reading stuff about the open source community goes like this:
Many open source licenses are irrevocable- once content is released under them, it is released under the terms of the license for anyone to use, forever.
If the license doesn't include such language however, and it included language that allows them to revoke the license, then yes, they could.
If it contains neither language that makes it irrevocable or language that explicitly allows its revocation, then they could presumably still break the terms of the license to revoke your rights to use the software if they're willing to eat the costs of breaking a contract. You could sue them for damages, and you might even win, but you wouldn't be allowed to continue using the software unless the court orders otherwise.