When designed properly and made of low carbon stainless(316L for example) than your chainplates out-live you easily. Proper design means that the stress levels in a worst case scenario are below 40% of the yield strength of the material. If that is the case, fatigue is not a risk, if low carbon stainless is used, corrosion
due to low oxigen is not a risk anymore.
Problem is that most chainplates are made of 316 and 304 ss and than glued airtight to another structure. If glued and bolted to an "aired" structure, they will live for life as well. The difference beteen 304, 316 and 316L cannot be seen with the bare eye, it needs to go to a lab. When changing your chainplates, consider upgrading to the low carbon steel
or make the construction such that the chainplate is in conenction with air as much as possible.