If martingale worked there would be a lot of dead dice sites but sadly I have tried to use it and end up losing all.
That is actually a good argument. People are still believing in martingale, so you could use that to convince them
People believe in it because in theory, it would work. The reality, though, is that there are two things stopping it:
1) Player bankroll
2) Site max bet allowance
One of these is going to be hit, as neither of these can be infinite. Should these be removed, it would change things, as you could lose millions of times in a row and still not have "lost" yet.