As-salamu alaykum
The first point to remember is that there is a huge difference in a sin committed in private between two individuals and another that is broadcast and forms a part of societal transgression where such practices become propagated and in turn, tolerated or are seen as acceptable. The focus remains not to spread immorality.
There are many communities in antiquity where lewd practices became the norm and were considered socially acceptable by the majority. The Quran discussed some of these communities. Even in the modern world, many practices which would be considered lewd from a Quranic perspective are considered socially acceptable by many.
With regards to the matter of four witnesses to prove the sin of 'zina', the important point to note is that the protection of the innocent against false accusations appears to be of greater importance from the Quran's perspective than the protection of the guilty (who will still be held accountable regardless, if not in this life, then the Hereafter). This can be seen in verses such as 24:13 ('Why did they not produce four witnesses? Since they produce not witnesses, they verily are liars in the sight of God').
Now of course, unless (1) the sin is readily committed openly or as in today is recorded / captured and seen by many as evidence, (2) the fornicators admit their sin voluntarily, (3) there is pregnancy outside marriage without rape or (4) a spouse refuses to provide an oath when accused (in the case of adultery 24:6-9), Zina in secrecy will be arguably extremely difficult to prove.
However, the bar of evidence remains high to protect against false allegations; the chastity and honour of a believer remaining paramount and to curtail lying false filthy speech against another which attracts its own severe punishment from a Quran’s perspective (24:4). The sin it appears, would rather be concealed temporarily than an innocent be wrongly accused.
016:061
"And if God were to seize mankind for their wrongdoing, He would not have left upon the earth any moving creature, but He defers them for a specified appointed term. And when their term comes, they will not remain behind an hour, nor will they be able to advance it"
I hope this helps, God willing
Joseph