That Allah is One and that His power is absolute was the subject in previous verses. In the present verse, it is being said that the Holy Prophet ﷺ “ was sent as the Messenger of Allah to all peoples of the world who are there now or will come in the future.
The word: كَافَّةً (Kaaffah) in: كَافَّةً لِلنَاس Kaaffatal-lin-naas: for all peoples) is used in the sense of making something universal and inclusive of all without the exclusion of anyone from it. The grammatical arrangement of the text required saying: لِلنَاس (lin-naasi kaaffatan: for the people as a whole) because the word: (kaaffah: all) is grammatically a hal (adverb) relatable to the word نَاس (naas: people). But, in order to put a clear accent on the universality of the mission of the Last Rasul of Allah, the word: كَافَّةً (Kaaffah: all) was set to come earlier.
The mission of a messenger or prophet assigned to all prophets sent before the Holy Prophet ﷺ was restricted to some particular people and particular geographical area. It is the peculiarity of the Sayyidna Muhammad al-Mustafa ﷺ ، that his prophetic mission is common and open to all peoples of the world. In fact, it is not simply for human beings, but is so for the Jinns as well. And then, it is not just for those who were present during his blessed time, but is universally applicable to all human generations to come right up to the last day of Qiyamah. And this very fact of the continuity and survival of his mission as prophet and messenger demands that he has to be the Last and Final of the prophets and that no prophet is to come after his appearance. The reason is that another prophet is sent when the law and teachings of the one before him are distorted or altered. Thus, a second prophet is sent to reform the people and reinstate them according to Allah's pleasure. As for the Shari’ ah of the Holy Prophet ﷺ and that of His own Book, the Qur'an, right through the last day of Qiyamah, the responsibility of its protection has already been undertaken by Allah Ta’ ala Himself. Therefore, it will hold on and survive till Qiyamah in its original state and there would be no need for some other prophet to be sent.
According to a narration of Sayyidna Jabir ؓ appearing in the Sahihs of al-Bukhari and Muslim and elsewhere, the Holy Prophet ﷺ has been reported to have said: 'I have been given five things that have not been given to anyone else before me: (1) That Allah Ta’ ala helped me by placing in my person such an awe as it is felt by the people from the distance of a month of travel. (2) That the whole earth has been declared to be a masjid and a purifier for me (in the religious codes of past prophets, their worship used to be performed only in particular places set up as houses of worship. Worship was not allowed outside their appointed prayer places, either in the open or inside homes. For the Muslim community, Allah Ta’ ala made the whole earth a masjid in the sense that Salah can be made everywhere - and in the absence of water not being available or its use being harmful, the dust of the earth was made tahur or purifier so that it could be used to make tayammum which becomes a valid alternate of wudu). (3) That property from the spoils has been made halal for me. Before me, it was not halal for any other community (among whom the rule was to assemble the war spoils collected from the disbelieving adversary and deposit it at an appointed place in the belief that some fire or lightening would descend from the heavens and burn it, and this act of burning would itself be the sign of the acceptance of their religious war. For the Muslim community, distributing the spoils in accordance with the rule enunciated by the Qur'an and spending it as needed was made permissible). (4) That I was given the station of the Great Intercession اِلشَّفَاعَت اَلکُبرَیٰ : ash-Shafa'ah al-Kubra) (that is, when no prophet would dare intercede on behalf of others on the fateful plains of the Resurrection [ al-hashr ], I shall, then, be given the opportunity to intercede). (5) That before me, every prophet was sent to his particular people - I have been sent as a prophet to all peoples of the world. (Ibn Kathir)