I was going to pass on your question, since the question implies that the Greek scholars had some ulterior motive in selecting the conjunction, “de”. The question further assumes that the word, baptism, refers to water baptisms. May I asked that one read again the broader passage for the text is not referring to water baptism but Spirit baptism.
The author of the Gospels could have used the conjunction, “kai” or “alla” instead of “de” The critical interpretative question is the context and content of the verses cited by you.
As one reads the context and context, there are several contrasting and or comparison words as mightier or more power than or whose sandals I am not fit to carry. John further states that I baptize you with water, but he will baptize you with the Holy Spirit and fire. As one looks at those words, the conjunction,
“de” can include the idea of and or but among other meanings. With those contrasting words between John and the one who was to come, the choice of “but” is not unreasonable. There is a contrast between the two individuals and their actions.
Acts 2 is the fulfillment of the promised Holy Spirit when he came on the Day of Pentecost. The cited text is not referring to water immersion or water baptism, but the coming of the Holy Spirit. Water baptism is a reflection of one’s belief that Jesus is one’s Savior and Christ. Acts 2:38 words are to repent and be baptized in the name of Jesus Christ for the forgiveness of sins. The result or proof of that faith is the coming of the Holy Spirit upon those who believe.
SUMMARY: The Greek scholars who translated the conjunction “de” based on the context of the passage. One is free to choose “kai” if one wishes but it does not change the contrast and comparison in the passage.