Yes, for some things you let them learn like that. When you’re watching them get ready to marry someone who is clearly going to screw them over, you do your best to open their eyes. Even then you can’t force anything on them, but at least you’ll know you tried. So will they when the S hits the fan.
When I was about to marry Mr. wrong, I laughed it off like a young idiot will. No one could tell me anything.
But you are right; you have to at least try.
Parenting is like water dripping on a rock. I love this analogy (and didn’t think of it myself): water drips on the rock and doesn’t seem to be having any influence at all. But over the years, you can see that the trickle of water has help to shape that rock. Keep dripping on that rock. Keep talking and parenting. Who knows what shaping is taking place?