Ensuring high availability for mission critical systems is costly. A greater emphasis needs to be placed on first determining what business functions are truly mission critical, scoping the business requirements for reliability and availability, and then designing the infrastructure to meet those requirements.
Just as important is the need to also ensure that appropriate staffing and processes are in place. System failures are more often a people or process issue than a technology issue. Any IT department that expects simply purchasing more-advanced hardware and software will deliver 4 or 5 x 9 availability is not seeing the whole picture.
Mission critical systems directly and significantly impact revenue, goodwill, health/safety, or regulatory compliance, and therefore demand a higher level of attention than non-critical production systems. Don’t assume staff are already following proper processes, or have the skills and knowledge they need to prevent incidents.