It’s not like there’s a giant pool of mining hardware waiting in the wings to take over... I think we can assume that all hardware is always in use mining something as long as marginal revenue exceeds marginal cost.

Ethereum is typically mined on GPUs so I would argue there really is a giant pool of hardware waiting to take over. On Bitcoin where ASIC mining is common that is indeed a concern, but consider even then, each hardware owner has stronger incentives to just lie about participating in such a strike while keeping the increased profits for themselves.