Why is it inadequate to treat wa-waḍaʿnā and wa-rafaʿnā as mere substitution?

Supplementary Practice 18: Rhetorical Analysis of Sūrat al-Ḍuḥā and Sūrat al-Sharḥ

• They report distinct favors joined by a shared purpose
• Neither contains wāw
• The second lexically negates the first
• Both are overt interrogatives