Optimizing tail latency isn't about saving fleet cost. It's about reducing the chance of one tail latency event ruining a page view, when a page view incurs dozens of backend requests.

The more requests you have, the higher the chance one of them hits a tail. So the overall latency a user sees is largely dependent on a) number of requests b) tail latency of each event.

This method improves the tail latency for ALL supported services, in a generic way. That's multiplicative impact across all services, from a user perspective.

Presumably, the number of requests is harder to reduce if they're all required for the business.