Generally, I think warm node can act like hot node, but slowly. And machine cost a lot saved.
But ES runs on Memory and require sufficient memory to process request regardless of how it fast.
Memory is the most expensive part in machine.
Imagine 10TB data, perhaps about 1TB memory will be required. we can not supply cheap memory if same query need to run on warm node. In that situation cheap disk or less cpu core can not save the total cost a lot.
Finally I think hor warm architecture will not save money than I expected.
How do you operate hot/warm node hardware?