There is only 1 simple answer to this. Don't convince them! Let them think it's a joke. Then, when it is costing them $20 for a loaf of bread and $500 to fill the vehicle up with Gas. You can look at them and say. I told you so...

You don't have to convince anyone to invest in bitcoin. If you are trying to convince someone, chances are, you're trying to make a profit off of it....
And that's not what Bitcoin is about. Nobody understands this.