I'm a bit embarrased to admit, but the original spark plugs in our old 740 B230 NA were were still running perfectly fine at 90,000+ miles. You replaced the plugs in that car at ??,??? miles and it only has 112K on it now, so my guess is that she hasn't put that many miles on them. But even so, I kind of like the sleeping dog approach, as long as it's still running good. There are many things that could happen down the road in the next few years (some we don't want to think about too much) that would render a plug change right now to be wasted. For instance, what if it blows a head gasket two years from now? There's a perfect opportunity to fix the spark plug problem at that time.
I vote to leave the plugs alone and concentrate on some of the other things that an 18 year old car might need. You have replaced the radiator by now, haven't you?