You still can sell X months access, if that's what you plainly state is being bought.

I don't think you could sell "for as long as the developer supports the game" specifically, since that'd be an illusory promise (no actual obligation if the product can be revoked immediately), making the contract unenforceable and the customer entitled to restitution (a refund).

"infinite support" is pretty much just "leave the customer with the product they bought working". There doesn't need to be any ongoing costs.