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.