You miss the point. The only reason to buy a diamond is that it is expensive. It doesn’t matter if you could buy the same thing cheaper. It doesn’t matter that there are better gems that are cheaper. You buy the diamond to show that you can afford it. You buy someone a diamond to prove how much they are worth to you.
Yes, you can buy a man-made diamond the same you can buy counterfeit branded clothes. That only shows that you’re tasteless and that you can’t afford the real thing.
The entire point of a diamond is that it’s expensive. People buy them for status. Otherwise there are lots of gems that are much more prettier, but they are not as expensive. It’s like saying people will stop buying branded clothes because unbranded clothes have the same or even a higher quality for a fraction of the price. Kinda misses the point.