Intermission: What's subarashiki about poetry anyway?
In classical japanese, the attributive form of い-adjectives ended in き, not い, and き is therefore an adnominal form. You'll run into this form all the time since it's still intelligible to native speakers, so it's no use hiding it.
Japanese stories like to use literary language features once in a while, just like english. Just as a novel might misuse "wherefore", so too will a japanese novel misuse <confusing literary term X>. While this doesn't apply to the き ending of い-adjectives, it does apply to things too advanced to be covered here, for sure.
All the same, you're going to have to acquire the misused versions of literary terms in order to understand them when writers misuse them. That's part of the experience of the language, how <feature X> is used, not just what it's "supposed" to mean.
Uneducated attempts to sound archaic are definitely not 素晴らしき, but the low-grade tone of voice you can get out of just the right use of old grammar, or the jokes you can make when characters know different parts of it, those are definitely 素晴らしき.
Classical japanese actually has a couple different categories that evolved into modern い-adjectives, but you don't have to worry about that.