Not OP, but I think you're missing one aspect- senior engineer time doesn't scale well, because there's not just wage but also opportunity cost of context switc

Not OP, but I think you're missing one aspect- senior engineer time doesn't scale well, because there's not just wage but also opportunity cost of context switching and taking them away from tasks that might contribute to increased sales or revenue. More sales also means needing to hire more people if the support load is constant per customer. If a chatbot can reasonably succeed at eliminating some of that workload without also driving existing customers away, it's a net win even if the budget between senior salary time and chatbot query is identical.