Tell them bitcoin gives privacy, and allows them to be in custody of their funds without a third-party. No one have control over their bitcoin as long as they keep it in their noncustodial wallet. If they are still not convinced, leave them alone because the are skeptical people and you wouldn't be able to convince them. Let them invest in altcoins and you invest in bitcoin, in the long run they will see the difference for themselves. Experience is the best teacher.
Experience is indeed the best teacher. Some people like to learn the hard way by themselves; personally, in some arguments with the kind of fact that most people present to distinguish while some altcoins are better than bitcoin, it is not even plausible.
In such a case, I just see that those people don't even know what they want to go into or what they are doing. The best I can do is ignore them because it will be a waste of time to drag myself into such a conversation, as it might make me look stupid at the end of the day.