انْخَفَضَ الْمَاءُ، وَلَكِنَّهُ لَمْ يَنْقَطِعْ.
Water decreased, but it did not completely stop.
Lakinna does not deny the initial report. It corrects an unwarranted implication a listener might derive from that report. Scarcity and cessation are kept distinct.
Replacing the contrast with wa-inqaṭaʿa would coordinate two asserted events instead of correcting a mistaken inference. The rhetorical value of istidrāk lies in that precise semantic relation, not in the mere presence of a second clause.