In practice, I'd have reservations about widespread manufacture of methane. Existing methane infrastructure is often leaky and methane itself is a far more powerful greenhouse gas than carbon dioxide. That isn't to say that manufacturing methane shouldn't be considered, but it's extremely shortsighted to say that methane manufacture is the only storage technology that should be considered.
Do you have a source for that? Also, where does the carbon for the methane come from? With iron/rust there is no carbon in the cycle.
Generally, anhydrous ammonia is a more practical medium where pure hydrogen won't do. It just needs hydrogen and air as input.
The cost of equipment will be coming down fast.
It needs, anyway, a supply of hydrogen and carbon. As hydrogen gets more valuable for other processes, its price might even go up. Extracting carbon from the atmosphere costs a few hundred dollars a ton; that cost might go down. The oxygen has to be stripped off, which costs energy you will hope to get back.