Have you ever considered the words of the Battle Hymn of the Republic?
As He died to make men holy, let US die to make men free
Union soldiers were fully aware they were fighting to end slavery. Historical revisionism is irrelevant.
No, you over-simplify the matter. Among the criticisms of Union generals McClellan and Buell was that they were soft on slavery and perhaps had intentions of restoring the Union to the status quo ante bellum. Some soldiers expressed dismay when Lincoln issued the Emancipation Proclamation, arguing that they were fighting to preserve the Union rather than fighting to free the slaves (and they probably used a pejorative not acceptable in polite society).
Certainly as the war progressed, the overthrow of slavery gained importance.