Like he said: "Beautiful, isn't it?". Also, like the other poster says, it's delivered so confidently like "I don't give a sh** what you think, this f***ing thing is my masterpiece".
 
Even if he acts contradictory, it didn't take much for him to backpeddle such an emphatic statement.
 
"Beautiful, isn't it?"
"...blah blah, I'm quiting"
"Oh, well, ya'know, I don't like it that much, eh, just destroy it when your done"
 
There was no deliberation about the positive and negatives, no arm twisting, Batman just completely backpeddled. I mean you could've had a scenario where Batman creates (hypothetically let's say it's towards the beginning of the film for sake of argument) the machine, Lucius quits, he realizes "I can't do this without him" and then reconsiders his positon. Then there is some actual catharsis, and some actual deliberation. Instead, as I say, it just ends up being the "God Machine" (almost literally). I kind of liked the machine, very interesting as far as comic book machines go, but I'd rather it have been fleshed out more rather than just crammed into the end.