OK let's say that the Battle of Leipzig becomes a glorious victory like Austerlitz. Does this give the allies a reason to pulled off the coalition ?
Depends. If Napoleon captures a king and a czar with their whole general staff and entourage and wins an Austerlitz-scale victory, Russia and Prussia are out of the Coalition, but the rest might stay and the French are still very stretched for manpower. Then again, the idea that Napoleon cannot be defeated by anything short of a force of nature will be even more prominent.
Strategically speaking, a crushing victory at Leipzig gives the French much needed strategic depth, and if the Prussians and Russians are out, Austria might not pursue further attacks on the Rhine, at least not until Britain provides some meaningful support. The smart move at this point would be to abandon Spain and wait for the British to make a mistake, such as giving Napoleon what he wants by offoring him an open battle, perhaps following a landing in Germany or the Netherlands (attempting to link up with the Habsburgs perhaps). If they do that and get promptly crushed by the French, Austria will be ready to make peace and maybe even Britain can be persuaded if Napoleon is willing to let Spain go.
Too bad it would be very unlike Napoleon to abandon the Spanish ambition, especially if the just defeated another coalition. Perhaps Talleyrand can work his magic on him? Or maybe he sees how stretched his forced and reserves are?