Thing is, a lot of these designs are much closer to deployment than they look - for example, the materials science of a low temperature molten salt reactor is entirely a solved problem - if you aim to run it at 4-500 degrees known and proven nuclear steels are just fine, and there is essentially no research left to do other than "Build a FOAK prototype towards commercial deployment". Which is a 4-5 billion euros project, and nobody in nuclear energy research has that kind of budget other than ITER. So instead you get 20 year research timelines where material engineers try to put together alloys that can hold up against neutron bombardment and operating temperatures of 900-1000 degrees. And stick those alloys into conventional research reactors and other esoterica aiming at building a technically sweet reactor.. in 30 years.  Because alloy research and computer simulations of theoretical high-temperature reactor designs can be done on a much smaller budget, you can demonstrate progress, publish and so on.
This doesnt mean that a fourth generation design couldnt be tested and deployed in < a decade. Because that could be done - it just means it would take a serious budget and political will enough to dictate that features that would be nice but are not essential for electricity production can take a hike, and that concrete is going to get poured and steel bent.
Which would probably meet quite considerable resistance from both black and green interest groups.

- as passively safe low waste reactor design which was honestly cheaper than coal would kill the entire economic rationale of coal, gas and green energy technologies stone dead.

The very high temperatures are intended for thermolysis of hydrogen. Not really a priority, because the hydrogen economy is certainly going to loose out to improved battery tech, and not really the brightest idea ever to begin with anyway. - you really want reactors who dedicate their entire primary energyout put to thermolysis of hydrogen on site? Because that can go boom.