I’ve had to budget a line of business app that runs on both z/OS and AWS. Reliability for both hit an asymptote somewhere around 2019. The opex price crossover happened somewhere around the same time, depending on the customer.

Nearly every installation site and their internal support director agrees that the z/OS installations are far less expensive to support, simply because the minimum level of experience for a z/OS engineer is about a magnitude more than a cloud engineer.