the adversative conjunction `but' in `poor but happy'
“貧しいながらも幸せ”におけるbutなどの反意接続詞 - 日本語WordNet
Linked forms in which the existence of "koso" caused the sentence to end with a classical imperfective form were more and more often accompanied by an adversative conjunction but remained for a longer time than the linked forms with an attributive form.
已然形で結ぶ「こそ」の係結びも、逆接の接続助詞を伴ったものが増えてきたが、連体形係結びに比べて後代まで残った。 - Wikipedia日英京都関連文書対訳コーパス