I did'nt suggest that diamond composites last only 18 monthes. My point was that the study on them only lasted 18 monthes and this is not a long enough time to draw conclusions.
I don't see why there would be any size limit on inlays. I thought the term inlay simply meant that a filling was produced in a lab and then cemented on the tooth. In general ,plastics are highly toxic and several people who are engineers or scientists have told me to get ceramic or gold.