1. It's not just the turbines which bring rotating mass but also the rotating parts of the generator. 2. One can actually use a synchronous generator only (without a turbine) to add inertia. These free spinning generators are called synchronous condensers. 3. Denmark already has some synchronous condensers, sited at substations (Bjæverskov, Fraugde and Herslev, there may be more). The Telegraph article (and the Slashdot summary) are also wrong on the statement "Denmark lifted a nuclear power ban". Currently there is just discussion if it would make sense to lift the ban, nothing more. See this article from Danmarks Radio (in Danish): https://www.dr.dk/nyheder/poli... Denmarks largest energy company, Andel, BTW is currently campaigning against nuclear power. So most likely the push is not founded in electrical engineering ("stabilize the grid") or economics but nuclear ideology.

Looks like this design uses an awful lot of material. I wonder if that really scales, both physically (weight) and economically (cost).

The US employment rate is not on a downward slope for decades. Quite to the contrary: https://tradingeconomics.com/u... It's considerably higher now (a good 60%) than in the 1950ties, 60ties and the 70ties.