If we are to believe the official bulletin of the Grande Armée, then N.'s plan to limit the escape route to a single bridge would have been a resounding success if it weren't for this stupid Monfort. And who is say that using the single bridge wouldn't have worked, especially since the only real problem apparently was it being blown too early.
If temporary bridges had been built, it might be a more difficult task to destroy them such that the enemy couldn't make use of them and make a pursuit. N. probably saw that it was better to limit the retreat to the single bridge and when needed blow it to completely deny the enemy any method of pursuit.
It would be interesting to know how many, if any, genie officers were still available at Leipzig. The reinforcement engineering group from the north was holed outside the city. The commanding Genie General was sent off to build bridges over the Saale, so, there was probably very few available other than Monfort and rather than have him busy building temporary bridges the priority was instead for cutting off the enemy. If the pictures and reports are correct bout this bridge job being so large, that's not something done in a couple of hours; it takes time and effort to properly set the explosives for a substantial bridge.
Looks like the entire issue of extra bridges over the Elster is, as Berthier so resolutely stated to Monfort, pretending to anticipate the genuis of N.