How much better would have Middle Earth been had assassination been not only attempted, but had been successful. But I guess Arda needs manure for growing the next generation...
Think about it. Angband gets thrown down (or, if pits, get thrown up) at the cost of many lives, places and things. As great as the Army of the First Age was, Melkor was able to raise a force that was about equal in strength; hence all of the collateral damage. Sauron finally falls (temporarily) at the end of the Second Age, though the cost was incredible. The Last Alliance made their way readily into Mordor, it took them seven years to finally bring the battle to a close, and during the siege there were many needless deaths.
Once, however, the good side shows up in such force that Sauron does not give battle as he doesn't have a sufficiently equal force, as when Tar-Calion comes to fetch him. But the death tolls are brought back up to suitable values when Númenor sinks and the Army of Ar-Pharazôn drowns.
So many deaths.
Thankfully, in the Third Age, Gandalf et al come up with a plan of assassination (as noted above) that minimizes the number of deaths on the good side while taking out the lead actor in the bad role. Aragorn's army of 7,000 is laughable in terms of Middle Earth "Let's Start a New Age Already" numbers, as noted in RotK. And yet, because for once deaths are minimized (and still many people and things pass away forever), the world starts off on a better footing than in Age-cusps past.
So, in short, had the Valar or the Wise taken out Melkor/Sauron with one shot each, the world as we know it would be probably overrun with Elves and Men and whatnot.
But it's not. How convenient...
