I read somewhere there's a school of thought that the way some halves were machined can cause IMSB failures and that if it hasn't failed after so many miles, then the probability of failure is low. I don't subscribe to it, I only mention it as the only possible justification for your what mechanic said.
Like everyone here advised, I would replace it if I were in your shoes and "in there" already. From what I understand, by the time you realize it's bad it's too late. I'd replace the bearing with the same one as the original. It lasted you this long, there's no reason a new one shouldn't last just as long or close to it. If you need help finding the same style bearing, youtube "burner's cars". He has a series of videos on this and considered several kinds of bearings before settling on his choice, and he gives some solid reasoning behind his decision. He also tells you where he sourced his, and it was VERY reasonably priced.
Here's the link to Burner's video I was referring to:
https://www.youtube.com/watch?v=IxdvSq_byZw&list=PLiya6G1pHhIYyzhlVzS_LwoIovrlv6_lv&index=16
And here's the link to the bearing specs: (Burner decided not to post his source, but instead posted specs and requirements)
https://burnerscars.blogspot.com/2016/08/sourcing-your-own-porsche-ims-bearing.html
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