To be fair, you're asking them to have solved a problem in advance that nobody knew that they would have.

Nobody had ever fired a rocket that strong. Their test firings were on par with the strongest previous rockets ever launched, and neither the test firings or those previous launches suggested that something like this was possible.

And it wasn't just them. They went through over a year of environmental review and mitigation efforts. To the best of my knowledge, the issue of how to handle the launch pad being blown over the local environment didn't come up even once.

Engineering is full of things like this. You learn that a problem is possible because it happens. Then you figure out how to mitigate it. That first time really sucks. But when it happens, you have to ask, "Was this something that should have been predicted?" And in this case I'm pretty sure that the answer is no.

The problem of rocket blast mitigation has been studied for 70 years. It is not a newly discovered thing.

They launched the biggest rocket ever made without any of the blast-mitigation techniques used on similar launches.

We generally do blast mitigation to minimize damage to the equipment around the pad. The calculation was that the equipment was built in such a way that it would survive. And indeed the equipment only suffered minor damage, and that was from the unexpected flying debris, not the blast.

I'm not aware of a previous case where the concreate launchpad shattered similarly to this. The closest thing to this possibility that I am aware of being discussed in advance was the possibility that the whole rocket could explode on the pad in a large fireball. Which existing blast mitigation techniques would not have helped with much.

Or even having publicly documented probable risk of forgoing blast-mitigation measures in their launch permit. Debris flew well outside the approved range.

That has to do with the size of the debris.

In a back of the envelope approximation, an object 10x as big in every dimension has 1000 times the mass and experiences 100 times the air resistance. And therefore flies about 10 times as far. (Only an approximation, there are other factors. For example gravity acts the same on both.)

Nobody anticipated the debris would include large chunks of concrete. And therefore it was able to travel much farther than expected.

It doesn't seem like a particularly hard problem, the max thrust was going to be twice a Saturn V and it's bigger than any conventionally launched rocket so far, and the next biggest rockets all have a water suppression system or massive flame trench.

No, they knew this would be a problem.

They extrapolated from a half-power test, and they extrapolated wrong. Turns out that the exhaust damage is not linear with power. Physical systems are not linear, imagine that.

I don't know.... I would imagine they have a lot of control. After all, the engines nozzels rotate. The entire areas was empty of people and they were aware of the high probability of an explosion - potentially before lift off. There was a ton of assumed/acknowledged risk, all of which was eventually signed off on by Nasa(?) and the FAA.