Regarding Lincoln; You think presiding over a war that cost the lives of over half a million American's qualifies someone as a good president? Really?
What exactly would you have done in his situation then?
You let them secede, that's what you let them do. Perhaps you forgot, most people have, but the US was founded on the principle of secession, complete with a revolutionary war to escape a central power. The colonists were paranoid of a central authority and placed strict limitations on a federal government with the Constitution, but secession was still an acceptable principle. Membership in the Union has many benefits and a lot of risks for leaving, but it is the "get out of dodge" card from tyranny and oppression from a central authority and was a tool to keep the size and scope of the federal government in check. Without secession, what recourse do you have in the face of egregious violations? The Union was always meant to be a voluntary membership. Regardless of the motivations of the South for seceding from the Union, Lincoln had no right to stop them. By invading the South, he effectively abolished secession and set the foundation for a monumental increase in the size of the Federal government.
In a strange twist of irony, the Federal government today is far more oppressive than King George III in 1776.
I've been trying to devote a few brain cells to pondering this scenario. It's actually an interesting thing to consider. What would America be like if Lincoln had done the opposite and legitimized a state's right to secede. For one thing, you'd have had zero stability. Slavery wasn't the only contentious issue back then. I bet you'd see lots of back and forth with states, the Union, and probably new unions.
"
I tell you, I won't live in a town that robs men of the right to marry their cousins."
Eventually, things would probably start to coalesce. Like minded people would congregate in near by states. States would produce generations of like minded people. All of this would probably boil down to 2 or 3 separate unions. It wouldn't surprise me if we wound up with two nations, split along what are our currently defined ideological lines. In some ways this might be considered a great improvement. In other ways it would be anything but (New Canada North and New Canada South). I suspect a unified USA is far more powerful than the sum of it's parts.
The biggest issue is that what we got was one big civil war to determine how things would play going forward. I suspect under the separatist union we'd have gotten many smaller wars, and possibly a big one anyway.
All of this is just idle conjecture, but it does seem safe to say that despite the ugliness of the American civil war, the result was a nation that was much stronger as a result of it.