(through the land and seen what happened in the end to those before them Allah destroyed them completely,) which means that Allah punished them because of their denial and disbelief, and saved the believers from among them. That is why He says,
وَلِلْكَـفِرِينَ أَمْثَـلُهَا
(and a similar (end awaits) the disbelievers.) Then Allah says,
Verse sixteen mentions four kinds of favours and blessings in the next world, that is, rivers of pure water, rivers of milk of which the taste does not change, rivers of wine which gives delight and rivers of purified honey which will be given to the righteous in plenty.
Worldly waters become polluted by getting mixed with earth, vegetation and other adulterating substances which make them stink. But in Paradise, there will be rivers having un-pollutable water. The river of pure, clean water that will not go bad in taste, smell or colour, (and will flow uninterruptedly without layers of dregs, scum or any other unpleasant- looking substances).
The people of Arabia, who had rejected the Prophet of God, were forewarned by him that they should not imagine that they would remain unscathed, simply because they were free to eat and drink at that time. They were completely within the grip of God and the proof of this was that if they persisted in rejecting the truth, they would be destroyed in accordance with the system of God. Things happened exactly according to the prediction. The upholders of Truth became dominant, while the rejectors of Truth were destroyed.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.