Pretty sure the only people that believe in that fantasy are those that still refuse to consider costs as relevant.
Even my Ramform concept, based off the patented survey vessel design, is still expensive, as a potential DIY in ferrocement, or if I can get the geopolymer formula to work out, and go commercial. $100/sq ft is still luxury home prices, and the TSI Barge is over 6X that.
Granted, once there is one built, that design can be repeated, generally for a lot less, but still...
Many of the 'save the world' ideas that fly in here really need to realize that living at sea ain't going to be a stroll in the park, hop on the scooter and get to Walmart, or whatever. Even locating just a few miles out, and you're talking about spending all day traveling back and forth, let alone the journey to and from some proposed International Waters doughnut-hole over 200 NM out.
If roaming seasteads ever happen, they will use the same means as ships, to deal with waves.
If we're talking about stationary platforms, then whatever it is will have to either be like the bottom structures, or take additional mooring, at tens of thousands of dollars per line and tens of thousands more, per anchor, regardless of type.
Nice thing about the bottom-structures is, they won't move and won't need anchors, or lines that have to be replaced, won't need maintenance, or cleaning, and as artificial reefs, they will encourage marine life to move in, creating an oasis in the seas.
So, how can these bottom-structures be built on the cheap and placed? How about the electrolysis seacrete? Use a solar-panel, a shaped wire-mesh form, and some sort of buoy to keep it up high, during production, then use an ROV and buoyancy control to position them, once they reach some target thickness.
No need to buy hundreds of thousands of tons of concrete, at all.