"Slavery was a necessary evil - Founding Fathers" - Tom Cottonhttps://www.bbc.com/news/world-us-canada-53550882
“But the fundamental moral principle of America is right there in the Declaration: ‘All men are created equal,’” he said. “And the history of America is the long and sometimes difficult struggle to live up to that principle.”
“That’s a history we ought to be proud of, not the historical revisionism of ‘The 1619 Project,’ which wants to indoctrinate America’s kids and teach them to hate America,” Cotton continued, as Kilmeade’s co-host Steve Doocy could be seen typing on his phone. “To believe that America was founded not on human freedom, but on racism. To think that slavery was not an aberration, but the true heart of America.”
Putting aside Tom Cotton for the moment, and his motivation for the "necessary evil" statement, which was to attack the 1619 Project -
Would the south have agreed to rebel against England if abolition of slavery was a pre-condition for a new united country". Of course not.
Would the northern states alone have rebelled. Maybe, but their chances of winning would not be good. If a condition of a future union of northern states alone was abolition of slavery, would they rebel? Probably not. In 1776 all northern states were slave states. It wasn't until 1804 that slavery was abolished in all northern states.
So, in 1776, it seems obvious that the founding fathers that opposed slavery would have agreed with the statement that slavery was a necessary evil for the founding of the United States. And many would not even have agreed that slavery was evil.