The grips are aftermarket, don't blame S&W for this one.
With modern manufacturing there should be no need for hand fitting. If I was buying a new car or power tool and learned the manufacturer had to file to fit and paint to match to make the parts fit to together I would run from that brand as fast as I can. Which at my age isn't very fast but still, I would be running.
Give me a gun where the parts are mass produced with enough precision to be interchangeable any day over one that has to be assembled by a skilled craftsmen that assembles the gun from sloppily produced parts.
S&W needs to improve their quality, I had a couple of problems with the Shield Plus I purchased a couple of years ago that should have been caught by the factory. But S&W needs to fix those sort of issues without going back to manufacturing techniques from 100 years ago.
Note to the OP: When I owned a S&W 69 I put S&W 500 grips on it and they really helped by both being wider so the recoil was more spread out and putting some rubber between the metal backstrap and my hand. I think those were rebranded Hogue grips that others have recommended.