Answer to why is John 4:39-42 recorded for us to read
V39: It shows how the Gospel is spread, from faith shared to faith received.
V40: It shows the beginning of the Gospel being spread from the Jews to the Samaritans, the first step toward the Gentiles, "the Savior of the world." V42
V41-42: It shows faith becoming personal to the Samaritans.
Answer to original question
I don't see this as ungrateful, degrading, or an insult. For anyone I have pointed to Christ, the greatest complement would be for them to say this.
And many more believed because of his word. 42 They said to the woman, “It is no longer because of what you said that we believe, for we have heard for ourselves, and we know that this is indeed the Savior of the world.”
(John 4:41–42, ESV)
What teachers want their students to only learn what they taught them? What teachers are not proud when their students grow and excel long after leaving their class? Every Christian needs to have their own faith. Not a faith dependent on other people.
Are we beginning to commend ourselves again? Or do we need, as some do, letters of recommendation to you, or from you? 2 You yourselves are our letter of recommendation, written on our hearts, to be known and read by all. 3 And you show that you are a letter from Christ delivered by us, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.
4 Such is the confidence that we have through Christ toward God. 5 Not that we are sufficient in ourselves to claim anything as coming from us, but our sufficiency is from God, 6 who has made us sufficient to be ministers of a new covenant, not of the letter but of the Spirit. For the letter kills, but the Spirit gives life.
(2 Cor. 3:1–6, ESV)
It is written in the Prophets, ‘And they will all be taught by God.’ Everyone who has heard and learned from the Father comes to me
(John 6:45, ESV)
“Behold, the days are coming, declares the LORD, when I will make a new covenant with the house of Israel and the house of Judah, 32 not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the LORD. 33 For this is the covenant that I will make with the house of Israel after those days, declares the LORD: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people. 34 And no longer shall each one teach his neighbor and each his brother, saying, ‘Know the LORD,’ for they shall all know me, from the least of them to the greatest, declares the LORD. For I will forgive their iniquity, and I will remember their sin no more.”
(Jer. 31:31–34, ESV)