Industrial synthetic diamond production is around since at least the 80s and from all we know about De Beers, they are certainly not trying to get the price down.
[ Prices below are arbitrary but illustrate why this is totally irrelevant outside of jewellery ]
De Beers relies on the fact that ordinary people don't understand diamonds aren't very valuable. After all, the diamonds they see sold by De Beers are expensive, so...?
De Beers plucks a $50 diamond from the mines, charges $5000 for it in jewellery and says that's symbollic of True Love, or whatever nonsense people believe.
But the people making diamond cutting saws do not care about True Love, so they are already buying whatever is cheapest, and they won't pay more than $10 for $10 of diamonds. To them that $50 diamond is worth... $50.
De Beers new plan is to make $50 synthetic diamonds, brand them, get across to consumers how these aren't "real" diamonds (hurting synthetic producers), that they're more "I wanna get into your knickers" than "True Love" and charge say $500 for them to emphasise that they're worse than its $5000 mined diamonds.
That makes no difference to the diamond cutting saw people, they still just want diamonds for their purely mechanical properties, and they'll continue to pay $10 for $10 of little diamonds.
De Beers hopes that this drives the synthetic diamond competition out of their market, and allows it to go back to charging $5000 for a $50 diamond in jewellery. The industrial market operates on a rational basis, there's no profit for De Beers there, it's all about the silly people paying $5000 to signal "True Love".
Considering the $800 per carat price tag for their artificial diamonds they want to charge a hefty fee for them much more than it would cost you today to get them form China or India.
Since the 80s but only tiny or imperfect stones. The true diamond age is when we can make large structural things with diamonds. Imagine an aircraft wing made from a honeycomb of diamond rather than aluminum.
Diamond is a very poor structural material. It is very hard, but also quite brittle; it breaks easily, is heavy, and is flammable. Additionally, airplane wings specifically need to be flexible to absorb turbulence (like the suspension system on a car), while diamond is very stiff.
It's brittle, but very strong. So strong that the brittleness is irrelevant for a huge number of applications, because you simply won't push them far enough. It will usually sustain more load than the same weight of aluminum.
About flammability... Technically it is flammable. If you heat it enough, it will react with oxygen. Steel will melt way before that point, I would be it would burn before that point too.
Diamond is stuff just like any other type of carbon. Solid diamond wings would be silly. But engineered microstructures of diamond could be very flexible where needed, and stiff where not. Diamond ropes, like bundles of carbon nanotubes, could do all sorts of things.