I've been doing this weekly for a long time. You think I don't know that? My own grandparents have come very close to falling for credit card scams numerous times and they would have had it not been for my instruction.
I don't just hand them the internet, I teach them everything I learned. And it's never a one time deal either. I've hit the setbacks you describe several times already too, and I still come back to help them. Often it's not about learning permanently but about helping them accomplish specific tasks. A lot of old people don't want to use the internet constantly like we do, they just want to FaceTime their grandkids once a week or look at sports scores and such.
I appreciate your concern that I know these things, but again, this is my life. I've been working on it several times a week for the past four years.