As I understand, they received substantial help from God in the form of plagues, etc, so after the defeat of the immensely powerful Egyptian empire, why did they not think the same would happen with Israel?
The commentators deal with this issue. The Bnai Yisrael as newly freed slaves were basically infants with the implications and behaviors that immaturity brings. I go into some detail at Beshalach: Why Israel sinned in the desert - childishness and immaturity.
The commentators have been pointing out that בני ישראל are compared to children when they went into the desert after the Exodus. Many, Jews and nonJews, religious and nonreligious, have used this analogy to explain what happened. In fact, that is why the term used is always translated as "Children of Israel".
As Yirmiyahu says in Chapter 2 pasuk 2
ב הָלֹךְ וְקָרָאתָ בְאָזְנֵי יְרוּשָׁלִַם לֵאמֹר, כֹּה אָמַר יְהוָה, זָכַרְתִּי לָךְ חֶסֶד נְעוּרַיִךְ, אַהֲבַת כְּלוּלֹתָיִךְ לֶכְתֵּךְ אַחֲרַי בַּמִּדְבָּר, בְּאֶרֶץ לֹא זְרוּעָה
2: Go, and cry in the ears of Jerusalem, saying: Thus saith the LORD: I remember for thee the affection of thy youth, the love of thine espousals; how thou wentest after Me in the wilderness, in a land that was not sown.
For example, Dennis Prager, a modern radio talk show host used this analogy in 1996 to explain why the Exodus is unique. Only then did Hashem actively and obviously intervene on a national level with open miracles in this way. Even the open miracles of the desert and the miracles in the time of Yehoshua can be connected to this. All other miracles were either hidden or individual. Even the miracle of the altar of Elijah was relatively restricted and for a specific circumstance. Actually, the fact that the sacrifice was burnt by "the fire of Hashem" was considered a normal response by a "god" rather than a miracle. Similarly, the sins of the Bnei Yisroel in the desert from the beginning to through the חטא העל (Golden Calf) can be attributed to the fact that as newly freed slaves they were indeed immature "children". Indeed, the חטא המרגלים (sin of the Spies) can be considered in the same way as an expression of childishness. The punishment for that can therefore be considered a necessity of our having to mature before we could progress to the next stage.