The problem isn't the subsidies. The problem occurs when the proportion of solar users increases such that the utility's margins become low enough to threaten the minimum budget required to maintain the infrastructure.

If you want to subsidize solar users to bootstrap that market and as a general social good, that's fine. But you have to structure it such that the utility still recoups the cost of maintenance. If they're paying market instead of spot, the paid price is not taking into account the cost of transfer and storage to the utility.