Please note that the Eth core devs have a contingency in place to account for hashrate changes which can affect the merge timing.

The actual merge is triggered by a TTD (total terminal difficulty) value is set in the code, this value is the difficulty of the entire Eth network up to that point in time which when it reaches a specific value triggers the merge to occur.

Due to fluctuations in hashrate (i.e. 5GB DAG size affecting some asics, miners moving on/off the network, etc.) the core devs have included a TTD override switch in the code. This allows the node operators to on-the-fly update the TTD value to a new number to account for hashrate fluctuations such that the merge still occurs around a desired date (i.e. Sep 15 or thereabouts).

The idea is that the core devs are continuously monitoring the hashrate and if it fluctuates to a point were it will affect the merge date to an undesired degree, the devs will publish a TTD override value which node operators will then use to update their nodes and thus adjust the merge timing to account for the hashrate fluctuation.