If you want a reaction that you can take energy away from the reaction, the reaction needs to create more energy than it needs to maintain itself. If you fail to take that energy away, the reaction will accelerate and your output will grow even further.

It is basic physics.

The only alternative would be to have an open system that runs on so little fuel that you need to feed it continuously. This has an entirely different level of problems, as now it will be impossible to contain the radiation to the reaction chamber and the containers of the spent fuel. Also with that you would need an entirely different design of how the radioactive material is held in place and how the reactions are controlled. The current way of adjusting how much you block with control rods probably won't work.

It is just impossible to have an exponential system like the nuclear reactions used in a reactor without active control measures. And active measures can fail.

For starters we are talking about concepts, not actually built and tested Reactors. If you have any connection to scientific research, technology development or engineering, you should know that between hypothesis, laboratory testing, prototype development, technology upscaling, establishment of production lines and finally long term operation routines there is a lot that will not be like expected, has to be revised, adjusted, scrapped, redesigned...

The history of nuclear energy is riddled with cases of hubris leading to disasters. It is evident that so far humans were unable and unwilling to give safety the proper considerations.

But from a practical point of view anyone with some industry experience would find the idea insane, that Small and Modular systems, so high throughput of small batches would increase safety. It is much more complicated to provide Quality and Safety checks in such an environment. Especially as these would be done by multiple for profit companies, the necessary oversight would be more difficult to provide for the regulation authorities, so in the medium run we will get Boeing like situations. Just that cost cutting and mingling will lead to reactors contaminating large swaths of areas on top of potentially killing hundreds of people.

So now you explain, why we should totally listen to the claims made by for profit cost cutting companies, that are solely based on concepts, without any actual field testing.

Because that was exactly the Titanic situation. People believed it to be unsinkable and decided to cut on costs for emergency measures. Reality proved them wrong on the first and last voyage.