Disagreeing again, I think this was a "camel's nose" strategy -- of showing the rubes only the camel's nose at first, and only later, to quote gay activist Marshall Kirk on the gay "human rights" agenda, his unsightly derriere.
Race was a hot-button issue in the United States, and I think Lincoln, while he was after 1854 committed to destroying slavery and anything and anyone who supported it, nevertheless was highly aware of the likely response to the idea of massive emancipation throughout the South, much less a brutal war against other Americans to achieve it -- so he took it off the table, until he was ready to impose it.
He may have justified the Emancipation Proclamation in political and diplomatic terms, but I'm still persuaded that it was actually the goal, and not just a tool. The vector in Lincoln's trajectory that pointed toward the 13th, 14th, and 15th Amendments was too strong to be the byproduct of a Clintonian politics of expedience.
Ending human slavery is a pretty good goal.
Walt