Uh, is this a DBWI? The South *did* get the Missouri Compromise repealed and Kansas territory opened to slavery in 1854. If you mean, what if they got it in 1850, the answer is that hardly anyone in the North was willing to do away with the Missouri Compromise in 1850--which is what made Douglas's 1854 argument that the Compromise of 1850 "superseded" the Missouri Compromise so infuriating to many northerners who had supported the Compromise of 1850. The fact that northerners rejected extending the Compromise line to the Pacific hardly means they thought it was obsolete for the territory to which it originally applied.
One important thing to remember: There were a lot more northern Whigs in Congress in 1850 than in 1854, and even the most moderate of them would reject such a proposal (as they did in 1854). Probably more northern Democrats would reject it in 1850 than in 1854 as well, without Pierce in the White House pressuring them.