Dams are precipitation energy capture devices. A very limited resource. Geothermal is heavily limited by groundwater and near-surface magma formations. It is a very rare phenomenon. Hydrogen is entirely limited just by wind and solar, which is vastly greater. It is thousands of times beyond what humanity currently uses in energy. So no, there's no realistic limit there.
The reason why it will "take decades for wind and solar to replace coal and gas" is because they are intermittent resources. The limiting factor is all of the infrastructure needed to make them work in a reliable manner. But they themselves are easily scalable to well beyond all human needs. Hydrogen as it turns out, is key to building the infrastructure needed to make wind and solar work. So in practice, hydrogen can be scaled at the fastest rate we can scale any green technology. Something that is vastly slower if not impossible to do with batteries because they are resource constrained.
Wrong. The most generous figures for hydrogen will recognize that fuel cells and electrolyzers are electrochemical systems. They are functionally batteries but built in a different manner. The maximum efficiency is potentially 100%, the same as li-ion batteries. This is particular obvious in large-scale implementations where waste heat can be reused. So even with no breakthroughs, we can easily reach 70% round-trip efficiency right now in those cases. In short, this is just a pointless exercise. You bring up efficiency in a field where efficiency is nearly irrelevant, and double down on that deception by lying about how efficient it really can be. In reality, hydrogen is just superior to lithium as an energy storage concept.