well, you have first to know why the god is sending the messengers to us
This is a good point to start.
according to my belief as there's still a valid message, the god won't send a messenger,
If you mean by a Valid Message 'Islam' then I disagree. I agree that Quran is left uncorrupted. But what I disagree is that the Message of Islamic Revelation is not the original Message that was intended by God and His Prophet, for after 1400 years, the Religion is turned into many sects, each made up their own Tafseers and Interpretations, and even some added their own Laws. You see some of Imams made Laws such as 'Soccer' is Haram. These are corruption in Religion, and therefore a New Revelation was required.
but when all messages had been corrupted or distorted by people then god will interfere and send to the mankind a prophet who will correct them their message,
The Reason that a new Revelation comes, is NOT limited to Corruption of the Holy Book, but also, corruptions in interpretations and Tafseers. Another Reason is God's Revelations are Progressive. In Every Age HE sends a New Book to teach more and more to humanity, and there is no limit for this, for He is Wise, Full of Wisdom, and His Mercy never ends.
as i believe that the god sent Jesus to Jews when they fully corrupt the Torah and no one on the earth was really a true believer as god wills so he sent them Jesus pubh to fix all of that, then when Christian did the same with gospel, the god sent prophet Mohamed puph to mankind to fix all of that,
This belief contradicts with Quran that confirms Torah and Injil that existed at the Time of Muhammad to be reliable. Moreover, according to many other Hadithes Torah and Injil could not have been corrupted for God rewarded the Jews who followed it till when Jesus came.
In fact, this belief that Torah and Injil got corrupted is a man-made belief, that was added later to Islam, by some of Muslim Leaders to justify Islam, which has not proof, and there is no single verse in Quran to confirm this if you read Quran and think about its message as a whole and carefully (without pre-judging!)
and as it was in the god's plan that prophet Mohamed is the last prophet so the god himself promises to preserve the last message of him to the mankind which is the Quran and so as it's preserved so why to send them a new messenger, the valid message is still preserved who wills to find it, it would be easy for him to find it, but the god is sending a man who refreshing the followers' minds from time to time by the main goals od that message.
The belief that after Muhammad there won't be another Revelation is a man-made idea that was created later in Islam. Quran declares God will send Messengers, and His Book in Writing will never be exhausted.
Let's do some investigation as what was intended by the Term 'Seal" in Quran.
The Word is "Khatam" in Arabic. Muhammad is called Khatam Al-Nabeen.
In Old times, "Khatam" was a stone on rings that was used either as 'Ornament" or as "Stamp" to Seal a document.
So, it is merely an interpretation that Seal means 'Last' or 'End'
In fact in Bible it is said:
"... on Jesus, God has set his Seal" John 6:27
(you can see the whole verse, I shortened it for brevity)
It does not mean, That God sealed His revelation and ended that with Jesus.
In this sense, 'Seal' means 'confirm'
One of the Missions of Muhammad was to confirm previous Prophets, hence the term 'Seal of Prophets"
In another sense, Muhammad was the Ornament of Prophets. Like an ornament on a ring, as an analogy.
For example Muhammad called Ali, 'Seal of Believers" ('Khatamul Auliya) that does not mean, He was the last believer.
Aisha, the wife of the Muhammad states;
[33]
"Say he is 'seal of prophets' but do not say there is no prophet after him."
[34]
See here:
Khatam an-Nabuwwah - Wikipedia, the free encyclopedia
The next thing to know is that there is a difference between Messenger and Prophet.
(Rasoul and Nabi)
The Nabis were the pomoters of religions. The Messengers were the ones with DIrect Revelation, and a New Book and Laws. So, Moses, Jesus, Muhammad were Messengers (and Prophet). But for example Aaron, and Somolon were only Prophets, and not Messenger.
Then after this we come to investigating Hadithes. There are infact Hadithes when Muhammad said, after Him no prophet comes. But we need to understand what that means within its context.
For example:
Narrated Abu Huraira: The Prophet said, "The Israelis used to be ruled and guided by prophets: Whenever a prophet died, another would take over his place. There will be no prophet after me, but there will be Caliphs who will increase in number."
The above Hadith says that, after Moses and before Jesus, there came a number of Minor Prophets (Nabi's) as a promoter of Religion of Moses. These Prophets were Aaron, Solomon, David,...etc. They did not have a Revelation from God with a New Book and Laws, but they had come to teach Moses Laws and guide Jews. So, the Hadith deals with prophethood within the dispensation of Islam. similar to Prophets within the dispensation of Moses. Muhammad said after Him, in Islam only Khalifs come, not prophets.
That does not mean that the Revelation of God ended, even as God said in Quran:
"O children of Adam, verily apostles from among you shall come unto you, who shall expound my signs unto you: Whosoever therefore shall fear God and amend, there shall come no fear on them, neither shall they be grieved."
al-A`raf 7:35
And according to following verse the Revelation of God has not ended:
"And if all the trees on earth were pens and the ocean (were ink), with seven oceans behind it to add to its (supply), yet would not the words of God be exhausted (in the writing): for God is Exalted in Power, full of Wisdom."
Luqman 31:27
Obviously the above shows the Quran is not the final revelation.