If the idea of sending Quran through Muhammad was to replace the previously sent messages because they got corrupted later that means humans did not have the proper message to get guided between the time of the oldest available manuscripts of the New Testament and the Muhammad's time. There were a few centuries between these two and a few generations of people lived and died during this gap. Wasn't god serious about this issue? Is god serious about people having the correct message with them?
Is the idea of the Quran the replacement of the prior Message?
If the idea of sending Quran through Muhammad was to replace the previously sent messages because they got corrupted later...
The intention of the Quran is not to replace but to confirm the Torah and the Gospel.
He sent down to you the Book with the Truth, confirming what came before it; and He sent down the Torah and the Gospel.
And the intention of the Gospel was not to replace the Torah but to confirm and fulfill it.
In their footsteps, We sent Jesus son of Mary, fulfilling the Torah that preceded him; and We gave him the Gospel, wherein is guidance and light, and confirming the Torah that preceded him, and guidance and counsel for the righteous.
This is also confirmed in the tradition of the Gospel:
Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished.
The intention of the Quran
We cannot know the intention of God beyond what has been revealed but we can and ought to think about the sense (knowing that we do not know it).
The Quran repeatedly states that it is a clear guidance, a more familiar example among many other passages:
This is the Book in which there is no doubt, a guide for the righteous. (2) Those who believe in the unseen, and perform the prayers, and give from what We have provided for them. (3) And those who believe in what was revealed to you, and in what was revealed before you, and are certain of the Hereafter. (4) These are upon guidance from their Lord. These are the successful. (5)
Possible reasons for the Quranic revelation
Conversion of the last People of Abraham that had lost orientation and followed a clear polytheism (Instruction to the Arabs)
Confirmation of the essential law of the Torah. The Torah had been revealed to the people of Israel. It has been subject to many interpretation by scholars. It was (and still is) unclear in particular to Christians who were not of Jewish origin to what extent they have to observe this law. The Quran gives a concise, partly simplified and partly altered instruction that is clearly directed to all people.
The Quran corrects a false interpretation of the Gospel where Christians confound Jesus with God (Instruction to the Christians)
The Quran confirms that Jesus is the Messiah (Instruction to the Jews)
Is God serious?
It's absolutely out of our competence to judge God. He is above all. He is the Creator, the Origin and the Destination of all, and He is the Judge.