If the transactions, and blocks are not valid/doesn't follow the consensus rules. Those transactions/blocks won't be relayed by full nodes. The energy spent will be wasted. That's the game theory.
Nope.
The incentive may help encourage nodes to stay honest. If a greedy attacker is able to assemble more CPU power than all the honest nodes, he would have to choose between using it to defraud people by stealing back his payments, or using it to generate new coins. He ought to find it more profitable to play by the rules, such rules that favour him with more new coins than everyone else combined, than to undermine the system and the validity of his own wealth.
That's the game theory.
Plus if you truly believe that your one node network will be adversarially safe from a collusion by a cartel of miners, because "game-theory", and that users shouldn't run nodes, then what are we in Bitcoin for? Remove POW too, and make it cheaper.
Firstly, I don't say users shouldn't run nodes, on the contrary I'm thinking of and advocating for mobile full nodes: every user one full node! I'm saying it is not helping bitcoin security to have a zillion full nodes, it is just about every single user to avoid being the one whose name is supposed to go first to the news. Full nodes do not form an institution to keep bitcoin safe! How people have managed to deduct such a stupid argument?
Secondly, PoW is essential for the game theory behind bitcoin and it is why PoS is shit.