Surely you can see that the P&W approach is only going to discover a subset of problems that will affect the engine in the wild. For example, engines don't run at 100% power during the full flight, and they go through heating and cooling cycles. It also doesn't address failure modes due to manufacturing problems specific to a single engine- for example, turbine blades are manufactured with serial numbers and you can go back and get detailed process and QA for that specific item to understand what went wrong and why it failed and how to not do that during manufacture again.

Then there's all sorts of complex failures that aren't addressed by single engines: " Prior to this crash, the probability of a simultaneous failure of all three hydraulic systems was considered as low as one in a billion. However, statistical models did not account for the position of the number-two engine, mounted at the tail close to hydraulic lines, nor the results of fragments released in many directions. Since then, aircraft engine designs have focused on keeping shrapnel from puncturing the cowling or ductwork, increasingly utilizing high-strength composite materials to achieve penetration resistance while keeping the weight low"

(I'm not disrespecting P&W- I'm sure they have more tests than just "100% power until it breaks, fix and repeat")