I second Oysterman's recommendation. If you weigh the ease of doing it when the boat is out of the water, vs in the water, a routine replacement every 3-5 years seems logical, regardless of stainless or synthetic. My keel pendant broke,the unrestrained keel punched a hole in the trunk, and I ended up getting the boat hauled, as I couldn't get it back on the trailer. Comparing that to the 20 minutes or so it takes while on the trailer makes preventive maintenance preferable. I am told that the "blue steel" synthetic line
is "chafe proof", but have not used it. I am planning on experimenting with a synthetic mooring cable this season.