Most things operating in industrial processes are going to have finite lifespans with the heats and stresses that are applied to them 24/7, plus in this case radiation. You're completely right about safety margin too. I used to run these simulations for mechanical engineers, and they'd always apply some safety factor. The challenge is is making sure that you're getting the most out of the material while still not compromising on safety.
All of that said, the analysis relies on tabulated data from the ASME code. I doubt they have the data necessary on radiation deterioration to do these detailed calcs. Assuming they don't, I think you're right that it would be prudent to retire them at this point.