Why would hydrogen combustion plants - which don't exist at the moment, don't have turbines on the market, don't have a fuel supply pipeline - be cheaper then current coal fired powerplants?
So your cost of generation already is - at minimum - at least as expensive as a coal fired powerplant, in terms of fixed costs for maintenance (and investment - who's building these when they can't sell the power from them?)
There need not be any fuel pipeline, since the plant can be built at the hydrogen storage site. The electrolysers will be there also.
There have been industrial turbines that burn hydrogen for decades. It's not some sort of exotic technology.
Yes, there are many parts here. And it's still cheaper than nuclear. Nuclear is pathetic in that way.
This is a common problem in the anti-renewable arguments. You pick a particular design for an energy system, argue it doesn't work, then (wrongly) claim no renewable energy system can work. But to reach that conclusion, you have to show that no combination of elements can make a system that works.
It makes sense to have multiple storage technologies with different performance characteristics. You want efficient, if somewhat expensive, technologies for short term storage with large numbers of charge/discharge cycles. You want low capital cost systems for ultimate backup, even if those systems are not as efficient.
For example, one could back up the entire grid with combustion turbines burning an e-fuel like hydrogen. These are massively cheaper per unit of power output than nuclear. Because we are not using them very often, the low round trip efficiency doesn't matter much. You want guarantees this won't run out? Make the storage caverns larger. This is already what we do with natural gas -- we store a good chunk of seasonal demand and count this being sized large enough to not run out.