Cost vs Reliability Trade-off
The non-linear relationship between availability and spend, which makes each additional nine roughly an order of magnitude more expensive.
Going from 99% to 99.9% typically means multi-AZ and redundancy — a modest, well-understood cost. From 99.9% to 99.99% means automated failover, no single points of failure anywhere, and often a second region: the cost multiplies. From 99.99% upward, the constraint stops being infrastructure and becomes change management, since most remaining downtime is self-inflicted by deployments.
The way to have the conversation productively is to price the failure rather than argue about the target. Revenue per minute of downtime, multiplied by expected minutes at each target, gives an expected annual loss. Compare that against the incremental spend and the answer usually becomes obvious — and it is frequently "the current target is already too high for this service".
The corollary: not every service deserves the same target. Applying the payment system's availability requirement to an internal reporting tool is a pure transfer of money to no purpose.