You're correct that as you go reusable, the economics shift away from capital costs to operating costs. However, it seems to me that when you consider first stages from a systemic perspective, it's not obvious that it's any cheaper. Yes, Hydrogen gives you much more impulse, and so for a given degree of propulsion, you can use less fuel by weight. However, LH2:
1. Costs 4x as much as kerosene by weight
2. Is only 11% as dense as kerosene, requiring much larger and heavier (and highly insulated) tankage to hold it, and larger and heavier turbopumps, combustion chambers, and everything else that puts weight on a rocket. So while your wet weight may still be less, your dry weight is generally quite a bit higher. T/W is always lower than for hydrocarbon systems.
3. This much greater bulk increases your drag losses if you're operating in an atmosphere; lower T/W increases your gravity losses.
4. Although hydrogen doesn't suffer from coking on the combustion surfaces the way that hydrocarbon engines do, it embrittelises (is that a word?) everything it touches, making between-flight inspections a much more crucial and delicate process. There are hydrocarbon engines which have been fired thousands of times between rebuilds, for hundreds of hours of firing time. Show me a hydrogen engine is even 10% as durable.
Of course when you're in orbit, neither drag nor gravity loses matter, and the nature of the Rocket Equation dictates that every ounce of gross weight really matters. So LH2 makes undoubted sense there. But the exact inverse is true when you're launching off the ground. Unless some fundamental breakthroughs have occurred which I'm not aware of (always possible), I will remain very surprised if SpaceX is pursuing cryogenic first stages.