That the Allies shouldn't attack Germany because there would be too many civilian casualties. Same with Japan.

Hmm. I believe some people did, and specifically the incident that most comes to mind as analogous to what’s happening in Gaza is the bombing of Dresden and that of Hiroshima, which many many people said should not have been done.

Oh no. The lesson from the Second World War was that it must not repeated. It is not supposed to be used as a justification for committing war crimes against civilians.