Status of superyacht Diamond?

There is no shortage of computer models of yachts that will never ever be built. I suggest that you wait to see a piece of steel, aluminum, or a hull mold pictured and titled before you start looking forward to a boat being built. If a yard that makes a lot of production run boats or semicustoms announces a new addition to one of their lines, and goes to the bother of putting a marketing launch together, well maybe then too. But if it's just a rendering and a handfull of press releases from nowhere? Well then you've really got nothing on your hands.

Until it's sold, paid for and delivered, I don't count it.

There is a very annoying job part between renders and laying down the keel: engineering, tech and build specs, safety&compliance... That is what makes a meaningful "yacht project", not design pictures.

As someone who actually worked on a retractable dome project for a big stadium... don't do it if you can It is cool, no argument.
Also gets cooler when you have to, first, pay for it - compared to the very same but without such fancy, then certify it (this was actually a perfect analogy from you, since both buildings and megayachts have to be), and then maintain.
It;s not crazy to like the design. Just might become when you do it Why, you guess, there's so few retractable dome stadiums in the world?

Also, subtle thing is, yachts are for travel. When are you supposed to use "beach club"? On anchor in some nice place, not middle of the ocean. So, why not use actual nice beach?

There are more than 100 with more planned, that seems like more than a few. With costs pushing the billion $ mark they are not exactly multiplying in number but are far from a rarity.

In comparison, making a retractable dome on a yacht is relatively trivial.

Because actual nice beaches within tender range and conditions are not all that common and hanging out on the boat is what most charterers pay for. Anyone can buy an airline ticket to a resort with a nice beach ... complete with crowds of people. There is absolutely nothing that can compare to a well appointed yacht with a "beach club" anchored in a place with water and a view that makes "nice" just commonplace.

We studied 30, so 100 sound just about right, worldwide. Out of 20 000+ there are, it's barely more then a few
As for the rest, matter of taste, really. My point is, you don't need a lot of sophisticated machinery to enjoy your time aboard, even at aft beach club.

When it comes to "big boats" that charter for $250K per week and up, it takes a great deal of sophisticated machinery operating invisibly in the background to make time onboard truly enjoyable. A successful "program" depends as much on the hardware as the crew and other "software" components of the experience.

True. I was viewing it from the owner's perspective (and my limited experience with that). 2 out of 2 megayacht owner/family I personally know enjoyed every single bit of gimmiks during the first-second year... and later started to really bother about the upkeep-to-effect ratio.
Same as with retractable stadium dome. Am I saying it's not spectacular?
I am saying that going in, over the years, the owner is bound to wonder whether it's spectacular enough to justify the costs. A lot of machinery just might sit there rusting, unused, for the bother of it. (1 of these 2, a wise old man, opted to store water toys right there at the aft "beach club" zone as opposed to separate sideloading garage because he recognized how bothersome lifing on/off will get over the years, even as a waiting time for him and guests, nevermind crew load. That's also 1 out of 2 megayachts I know where water toys are still used every possible time. Huh!)
All this is probably easier on charter boats, where, as you say, one can see the wow-factor working as a proper lure.

P.S. Also, while we're at it, what is that "lot of sophisticated machinery on the background necessary for guest enjoyment on a 25mil+ yacht"? I mean, aside from the obvious - lightning, toys crane, elevator, climate (which is not really a machinery...)?

You are making water and ice, you are controlling the indoor temperature and humidity, you are controlling the roll and pitch, you might have a dynamic positioning system, you've got all sorts of complicated electronics to make sure you don't strike ground or another vessel and die (rather unenjoyable way to end a vacation I suspect) you've got the engines to go at all, if it's a sailboat you probably have hydraulic systems to run the winches, you've got mechanicals to raise the anchor and probably to snug the dock lines, bow thrusters to make getting in and out of tight spaces less uncomfortable, etc. etc. etc..

I suspect that the glass atrium is not beyond the scope of possibilities, thought I am fairly certain that it would have a lot more structure and a lot less glass by the time it actually got built. That is the way of things, the renderings always show the prettiest configuration that doesn't look impossible to someone who probably has no experience building anything outside of a computer. I don't think a retractable stadium dome is the right level of complexity to make the comparison. A retractable stadium dome is a really profoundly large piece of engineering, where as this whole atrium door should be a few thousand pounds but the time you peel the protective coating off the glass.

The real point here is that how cool you think something is shouldn't be an indication of whether or not it gets built, unless you have the money and are going to build it. There are very few individuals in the world having yachts built at the top end of the market, and inconveniently they all have their own tastes that do not necessarily coincide with our sensibilities.