Besides, this is literally the first time ever it has reached temperatures this high - and it has been open for 84 years. I bet they also "failed to design" for hurricanes, cataclysmic floods, and volcanoes!
Coping with higher maximum temperatures means more cracking and damage at very low temperatures, so it is not necessarily a case of just building in more buffer at the top end but a trade off between the cost and frequency of events like this and the cost of dealing with the effects of winter.
Given that this had literally never happened before and the frequency of similarly hot events was also lower in the past, it’s perhaps unsurprising that the trade-off landed where it did, even if it may well be in for a rethink now.