至关重要 strikes me as a four-character idiom instead of a Chengyu, and it didn't even make an entry on 现代汉语词典 or ZDic.net. However, that couldn't explain the fact that the specific character combination
至关 is exclusively used in this phrase, and this phrase has an incredibly high uniformity, in other word variations of this word almost don't exist.
Usually four-character idioms can be modified to suit the context and people actually do that for colorful speech. But searching 至关 on Google, all results are 至关重要. Searching 至关必要, 至关需要, 至关基本, 至关有效... any possible application yielded zero result.
I am not convinced by it - not just the websites don't look too credible, but the book was first published in 1986 and wasn't popular until it won literary prize in 1991, however, back in the early 90's when I attended school, everyone knew the idiom 至关重要 as a common Chinese word. At that time there was no internet and I don't think that novel could have gotten that popular in such a short time frame.
So my question is, what's the real etymology of 至关(重要) and why there is no variations albeit it's commonly used? Was it a 'prescribed' word by some central committee? Did it gain popularity because some famous person or TV commercial used it in a catch phrase?