As @merzkrashed mentions the FDA “results of a process cannot be FULLY verified by subsequent inspection and test” and ISO “processes for production and service provision where the resulting output cannot be verified by subsequent monitory or measurement”. So if you can’t verify every requirement or verify every single item produced, process validation is needed. As you mentioned, prospective validation can be expensive and FDA has 3 phases; Installation Qualification (IQ), Process Performance qualification, and Product Performance Qualification. I think medical devices, especially class III, would require validation. Although it may be costly to do, in the long run it will end up being beneficial to the company because you end up paying more if these requirements aren’t met.