Indeed - the approach that CIYAM uses is known as a "hash chain" (which functions like a PRNG meaning that the next best choice of minter is random).
Without some external source of entropy, why can't participants just preimage the PRNG to know when to mint so they always win when they do?
The question is whether there needs to be any "reward" at all (assuming that you don't require increasing costs of hardware and electricity in order to run a full node).
As I see it, unless there is a trust based model like PoS, rational participants do not act in the benefit of the chain as a whole because the timely confirmation incentive is ill defined in a system with no rewards.