You call IBM using your support contract to bring spares or use the chance to replace with latest model (they are still updated)
I guess this is one of the reasons no one ever got fired for buying IBM.
No, it's just normal case of "the system is still in production and the only reason you're on ancient hardware is because you're not upgrading", same as if you ran business-critical component on random 486SX assembled by then-CEOs nephew.
AS400 is very much alive and stable platform with new release and adequate support available. It just ain't sexy and that was the main driver for declaring it legacy.