My brother-in-law was slated to be in the first wave in the invasion of Japan. He'd probably have died, as would many, many others.
If you scale up from the example of Okinawa, Japanese deaths would have probably numbered in the millions.
As it was, the bombs killed a couple hundred thousand -- a hell of a lot of people, but fewer than would have died otherwise.
And lest we forget, that invasion was already a scheduled event. The "saved lives" argument is valid.
No invasion was necessary. The Japanese were ready to surrender, all that was required was that they be offered halfway decent terms. And they were after Hiroshima and Nagasaki was used to demonstrate the atomic bomb to the Russians.