I'm not assuming a zero-sum. The concern still holds without a zero-sum situation, as long as some commonly used sources of energy are not fossil-free.
>but that's unlikely because fossil free energy (solar specifically) is the cheapest type of electricity generation we can build today.
Higher demand can increase prices. Usually this wouldn't be a problem, but considering how big the iron/steel production sector is, the impact of the electricity used for the H₂ be measured, not assumed.
>Furthermore, the hydrogen can be electrolyzed at times when the supply of fossil free energy exceeds demand, thereby actually improving the economics of intermittent renewables by increasing their overall utilization, and hence incentivizing building more if it.
That is actually a fair argument. Unlike the above.