I think it could be possible for the reward to be other than "coins".
There is no such thing as a "reward without money price [direct or indirect]". It is useless.
It might be useful to discuss why you think this is the case?
It is certainly true that valuing the PoW expended by the miners is essential for knowing where the break even point for a double spending attacker is. I.e. the point at which what they gain from the double spend == their lost block reward from orphaning their original blocks.