وَيْلٌ لِلْأَعْقَابِ مِنَ النَّارِ is a forceful warning. It is followed immediately by أَسْبِغُوا الْوُضُوءَ, a concrete command to complete the ablution properly. The point is correction of a particular failure, not merely alarming the listener.
The teaching sequence matters: the mistake was observed, its seriousness was identified, and the correct action was explained. When essential duties are being neglected, good instruction combines accuracy with an understandable way to improve the practice.