Fault some hyperbole, but Lincoln was saying “Look at it shrink, unless we stop that.” And to tell the truth, most of the reconquered areas, SANS slavery now for 150 years, are the greatest boon to that ideal today!
But even if you look at it like a slippery slope argument, it wasn’t shrinking. Because it’s not as if the Confederacy was a communist dictatorship. (It turned into a sorta dictatorship as Billy Yank killed it, but then again so did the North without the excuse of fighting for its existence.) Had they remained split there would’ve been just as much government by the people as before, only in two instead of one country.
Actually, a better argument could be made that Lincoln was fighting against government by the people in forcing Southerners back into the union against their will.