It wouldn't have brought a quick end to the war. We were GOING to win the Pacific War and had fought at great cost of lives on both sides to the point where an invasion and complete victory WERE coming. It was, because of the mindset of Japanese at the time, going to cost thousands and thousands of lives from both sides but the end was not in doubt when we used the nuclear option.
The nuclear option was ONE WAY to stop the war without costing so many more lives. Wars come down to:
Are all these lives (ours and our enemies) worth our objectives?
Are there ways to demoralize and "shock and awe" our enemy to get them to give up sooner with less lives lost for everyone, especially us?
The answers, with the nukes vs Japan, was yes and yes.
This isn't "capture the flag" in your backyard where running through the house and ambushing your opponent via coming out a back bedroom window is "not fair." You do what needs to be done to get it over with whether it's conventional or not, cruel or not, horrible or not to save as many lives, yes...... preferably your sides lives..... as possible.