It survives only inside a handful of fixed openers, and these lines show the main ones: 장담컨대, meaning I guarantee; 단언컨대, meaning I state flatly; 예상컨대, meaning my guess is; and 바라건대, meaning I hope. It puts weight behind whatever comes next, which is why advertising slogans and speeches love it. Do not try to build new phrases with it, because nobody uses it outside these set forms.