This was a question posed to Greg Koukl on his "STRask" podcast, and the 'start again' was posed on the basis of God knowing that he'd be wiping out all except Noah in a few centuries.
His answer was a little like the curate's egg: good in parts. The not good parts arose from his Calvinism and his view as to 'long ages' defining the history of revelation and thus the cosmos.
As a result he missed the obvious point, which I discuss.
Initially, the question seems to presume a couple of things that, to my mind, run against the Bible's data on creation (with respect to mankind's place and role, and God's action in creation) and on biblical anthropology.
Firstly, it seems to consider a cosmos that is disconnected from the creatures in God's image and that their fall affected only themselves. Subsidiary to this, it seems to presuppose that there was death before the fall, contrary to the God who found everything very good in a world not yet marked by corruption. Even animal death would be very much a mark of corruption. Upsetting, as it often is even today, rather than a matter of joy!
Secondly, it seems to trivialize the scope of sin and God's true relation to his creation and its creatures in his image: that we are made for fellowship and the first couple are the only ones by whom that unique status is unveiled. It would hardly be significant if there was a 'start again' option! God made them for fellowship, and now he goes about restoring the breach.
The notion that God could 'start again' with a new couple fails to recognize the effect of the fall on the cosmos. Paul tells us that the entire creation is groaning (Romans 8:19-23). So post fall, any new couple would no longer be in the pristine creation, but in the marred creation, subjected to futility and corruption: death now the unwelcome feature.
There would be other depredations of sin affecting the entire cosmos by virtue of Adam and Eve being God's image bearers and vicegerents. A new couple would be aliens at best, in a world not fitted for them and be either immediately frustrated, not knowing the world for which they were made as it has been damaged, or quickly become part of it. At worst they would conceivably be completely incompatible with it!
The original couple who fell being God's image-bearers are not just disposable and replaceable. Because sin has entered there could be no starting again. Yet they are rescuable, and God shows both his love and his power in that he is not outdone by sin, but rather overcomes it, as he promises them. A benefit of this is our further encouragement that God is truly the King of Creation and not a Zoroastrian bit player. God is not equally opposed by the 'not-God' of sin, but overcomes it. He does not tolerate death, but in Christ defeats it.
Following on, then, the gross condition of Noah's time was cleared for the line of the Messiah to be established in the. God let conditions play out until they needed to be wiped away for his plan to unfold.