two evils. A land invasion of Japan would have been disastrous, both for the Americans and for the Japanese. Yes, the Americans and the Allies would have eventually won the war anyway, but it would have dragged on for at least another year, maybe even longer.
Military experts estimated at the time that a land invasion would have resulted in at least 1 million American casualties, maybe even more, and millions more Japanese deaths. Japan was quite ready and willing to fight to the death.
The decision to drop the A-bomb probably ended up saving millions of lives, both American and Japanese.