Most of the frames you are referring to as 'monocoque' are actually several pieces that are lugged and glued together. The lugs are not apparent after the finishing is done and there are fewer pieces than frames like the C60 have...but they are not made as a single piece and popped out of a mold.
Construction techniques have been refined and will continue to be refined....but calling them monocoque is a misnomer.
You're technically right, of course, but I think the distinction here is between frames that are made from prefab tubes/cured-in-place lugs and monocoque/quasi-monocoque frames.
Tube-and-lug construction is less structurally efficient than monocoque/quasi-monocoque construction, and a company that can buy a set of 5-7 molds for one model can often afford to pay engineers to come up with a really light-yet-robust laminate schedule. One example of this is Trek and its Emonda.
The combination of structural efficiency and mature laminate schedules mean it's easier to build a sub-700-gram frame as a monocoque/quasi-monocoque than it is to build one with tubes and molded-in-place lugs.
I expect tube-and-lug construction to become even more of a specialty for custom builders (who need the inherent flexibility) and sentimental legacy brands like Colnago. The driving reason is aerodynamics. If you're designing a mold anyway, it's straightforward to make the frame more aerodynamic than you can with tubes bought from ENVE and molded-in-place lugs.
There's room for both in the marketplace, of course. But a few years ago, I drooled over custom round-tubed Crumptons. Now, I'm holding out for a disc-braked Madone. I don't need custom geometry (though I'll be paying too much extra for Trek's H1 fit) and the Madone's combination of comfort and aerodynamics at a reasonable weight is hard to beat.