I just laugh and say, "OK."
Time has proven them wrong, and it will continue to prove them wrong. You don't have to say anything.
Discussion
I usually donāt mind ignoring people who are set on their silly ideas, but when I know theyāre spreading FUD and discouraging others from digging deeper on Bitcoin it rubs me the wrong way.
Don't try to convince them, if anything, try to convince the audience that might read along. That's usually less triggering.
Lately, I have been trying to engage in conversation with FUDsters for the sake of counteracting their effect and sharing useful information for anyone reading. It does help to focus on that.
But given everything Iāve been seeing and experiencing in Bitcoin circular economies around the world lately, it irks me a little when people who have never left their neighborhood spread FUD about something they know nothing about other than what theyāve read on the NYT š
I can understand that. People in first world countries that criticize a network that's literally saving people in third world countries. People are able to transport their wealth and get out and you want to stop that and call it useless? Monstrous.
Itās an interesting problem. In the past I heard bitcoiners referencing Albert Jay Nockās essay, Isaiahās Job, to build up this idea of the project aiming for a remnant. The project idea has to āfesterā and ācorruptā for awhile. 
https://mises.org/library/isaiahs-job