I agree but that’s a highly misleading number because there’s a conflation of scenarios. Capacity is one thing, useable capacity at the requested discharge rate is another. You can’t pick and choose which numbers to use.

The combination of duration class and capacity is useful, though. 1MWh of 10-hour storage gives you a very clear idea of usable energy.

What if the discharge rate versus cost plot is mostly flat?

Then typically you'll run it at a higher discharge rate, and has something cheaper with a higher capacity/discharge ratio.