Answer
Part of why the platform has this staying power is architectural. Db2 for i is built into the operating system rather than bolted on, object-level security and QSECURITY settings give administrators fine control over who can do what, and journaling supports both data integrity and disaster recovery in ways many newer stacks still have to assemble from separate tools. That reliability is exactly why so many finance, distribution, and manufacturing workloads never got a compelling business case for replacement.
The real risk is not obsolescence, it is drift. A shop running an old, unsupported IBM i release, behind on PTFs, with hardware approaching end of service life and a shrinking bench of RPG or COBOL programmers, carries meaningfully more risk than a shop on a current release with a documented succession plan. Buyers evaluating a company's IBM i estate, whether for an acquisition, a vendor relationship, or an internal audit, should ask for the current OS version, PTF currency, hardware age, and how many people actually understand the core applications well enough to change them safely. Those four answers say more about platform health than the AS400 label ever will.