Slavery wasn’t even an issue until later in the war, and it had nothing to do with freeing the slaves for any moral purpose. There were two main reasons for the Emancipation Proclamation. 1-To disrupt the South’s economy because of the thousands of slaves that would escape to the Union’s lines. 2-To keep Europe out of the war. Europeans were addicted to American cotton. Britain and France supported the South. However, once the reasons for the war included slavery; the more enlightened Europeans no longer supported the South.
Also worth noting is that the EP did nothing to free slaves in loyal border states.
Another thing worthy of note is the majority of Northerners didn’t want the blacks. When they decided to free the slaves in D.C., Lincoln looked for places to resettle them. They looked for places in Latin America to send the slaves once they were freed. The newly freed slaves in D.C. rioted when they heard the news. The representatives trying to sell Lincoln land were scam artists. The idea was eventually scrapped.
Slavery was also going to die anyway. In the Northern parts of the South it had already become unprofitable to keep slaves. Tobacco was dying. Many slave owners in the Upper South were freeing their slaves leading up to the Civil War. I really don’t think the institution of slavery would have lasted much longer if the Confederacy had won the war or if the US just let them leave peacefully.