Says who? I think they are all about the way God reveals Himself.
God's way is always to help his religion with his friends. Although Prophethood ended, the witnesses and Imams for each day and age continued.
Imam Mahdi ultimately is the one who will unite all people under God's Wilayah. But the Quran being protected remains as a sufficient clarification and reminder.
So while revelation ended, his sunnah of guiding did not, just as he sent a succession of messengers after Moses, he appoint a succession of Imams after Mohammad.
The world of course needs an Imam, but the world needs to recognize their need of God and his chosen guide...
The logic that the believer showed regarding waiting for Moses and saying there is no Messenger...does apply to this nation...but as Prophethood ended, it means we should not say God will not appoint an Imam as God never stated he ended Imammate. So in the same way they saying there is no Messenger with no proof...stating there is no Imam after Mohammad with no proof and that God will not appoint another guide is the way of misguidance.
God will unite humanity on his guidance surely - through Imam Mahdi, so he is like Musa freeing the world from oppression and the Pharaohs of their time.
The Sunnah of God has not changed. And as for the words of God, there is another mystic interpretation, that this doesn't even referring to God talking but his revelation in his light and his words in his light, as we say that God made Imams the mines of his words in ziyarah.
From misbahal hidiya:
Know, O spiritual friend, may God give you success in
attaining His pleasure and place you and us among those who
witness His names and attributes, that this vicegerency is one of
the greatest affairs of Divinity, one of the noblest stations of
Lordship, the gate to the gates of manifestation and existence,
the key to the keys of the unseen and the visible, and the station
of withness (maqām al-indiyyah), in which lie the treasures of
the Unseen, which no one knows except Him.46 Through It the
names became manifest after they had been inward and the
attributes came out after they had been hidden. This is the
greatest veil by which every young and old is annihilated and by
whose presence every rich and poor is absorbed. This is an
endless space, above the Throne (al-arsh), in which there is
neither vacuity nor plenum. These are the august glories of His
face; were they to lift the veils of light and darkness, they would
burn up everyone whose eyesight perceived it.47 So glory be to
God, how great is His worth, how splendid His station, how
noble His face, and how lofty His sovereignty. He is the Allholy,
the Glorified, the Lord of the heavens of the names and the
earths of creation. Isnt it strange how a bat wishes to praise the
sun of the rising suns, and (how) a chameleon desires to describe
the bright overwhelming dawn?! How incapable are the pen and
the words and how dull are the heart and the tongue!48
قل لو آان البحر مداداً لكمات ربّي لنفد البحر قبل أن تنفد آلمات ربّي.
Say, If the sea were ink for the words of my Lord, the
sea would be used up before the words of my Lord were
exhausted.49
How should then be the case with the origin of the words
and the source of the signs! For the oceans of existence and the
pens of the world of the unseen and the visible fail to describe
any of His self-disclosures. How splendid is His proof and how
great His sovereignty!