I. The civil war was not inevitable: Lincoln caused it by his election and choices
War happens because politics fails. Lincoln's Republican party was perceived as anti-Southern, as evidenced by its immediate promise to impose import tariffs that would hurt the South's economy (to the benefit of the industrial North) and long-term commitment to the abolition of slavery. Although not the most radical candidate the Republicans could have nominated (more on which below) Lincoln was thus an extraordinarily divisive presidential candidate who was elected entirely by Northern voters. The Southern states launched their secession as soon as they heard Lincoln had won because they believed he (and his Republican party) intended to pursue an anti-South agenda that would further cement their political marginalisation within the USA. Perhaps reflecting his party's political ignorance of the South and his administration's inexperience, Lincoln seems not to have understood this. He therefore dramatically underestimated popular support for secession in the South and assumed a show of force would quickly crush it.
It is reasonable to suppose that if a different party's candidate had won the 1860 election, secession would not have happened then (or perhaps at all), despite the long-running tensions between South and North. Moreover, it seems likely that a different president could have managed any secession crisis without resorting to war because that president would have had the trust and legitimacy in the South that Lincoln lacked. (For example, one of Congress's various efforts to resolve the secession crisis by constitutionally protecting slavery might have succeeded.) Or the South could have been allowed to secede, on the same principle that the colonies had claimed in declaring their independence from Britain. The South's goal was just to leave the USA, and this was perfectly compatible with the continued existence and flourishing of the country that remained (though perhaps not with its pride). It was the South's choice to declare independence from the USA, but it was the choice of President Lincoln's government to go to war to force them to stay.
II. The war was an astonishingly costly way of achieving abolition
The costs of the civil war were 620,000 soldiers killed, 475,000 wounded and some $20 billion in 1860 dollars in direct and indirect economic costs (about twice annual GDP). The benefits were that approximately 4 million slaves were freed by the North's victory in 1865. Breaking that down, for every 7 slaves freed, 1 soldier died and $35,000 in economic value was lost (as a share of GDP, equivalent to $155 million today). Moreover, 'freedom' did not mean equality. Due to the federal government's abject failure to enforce the Reconstruction Amendments (because retaining the Southern states meant installing a large block of white supremacists in the national legislature), African Americans continued to be deprived of basic political, civil and social rights for another 100 years. In 1900 for example, life expectancy for African Americans was a mere 33, about the same as in 1860 and 15 years less than for Whites.
III. Lincoln's war was not necessary to end slavery
I will say then that I am not, nor ever have been, in favor of bringing about in any way the social and political equality of the white and black races, [applause]-that I am not nor ever have been in favor of making voters or jurors of negroes, nor of qualifying them to hold office, nor to intermarry with white people; and I will say in addition to this that there is a physical difference between the white and black races which I believe will forever forbid the two races living together on terms of social and political equality. And inasmuch as they cannot so live, while they do remain together there must be the position of superior and inferior, and I as much as any other man am in favor of having the superior position assigned to the white race. I say upon this occasion I do not perceive that because the white man is to have the superior position the negro should be denied every thing. I do not understand that because I do not want a negro woman for a slave I must necessarily want her for a wife. [Cheers and laughter.] My understanding is that I can just let her alone. I am now in my fiftieth year, and I certainly never have had a black woman for either a slave or a wife. So it seems to me quite possible for us to get along without making either slaves or wives of negroes. I will add to this that I have never seen, to my knowledge, a man, woman or child who was in favor of producing a perfect equality, social and political, between negroes and white men. (Lincoln's speech at the Fourth Lincoln-Douglas debate, Charleston 1858)
He was preeminently the white man’s President, entirely devoted to the welfare of white men. He was ready and willing at any time during the first years of his administration to deny, postpone, and sacrifice the rights of humanity in the colored people to promote the welfare of the white people of this country. (Oration in Memory of Abraham Lincoln, 1874)