I don't know if these things make it robustly good, but some considerations:

• Raising and killing donkeys for their skin seems like it could scale up more than the use of working donkeys, since (1) there may be increasing demand for donkey skin as China develops economically, and (2) there may be diminishing demand for working donkeys as Africa develops economically. So it could be valuable to have a preemptive norm/ban against slaughtering donkeys for this use, even if the short-term effect is net-negative.
• It is not obvious that working donkeys have net-negative lives. My impression is that their lives are substantially better than the lives of most factory farmed animals, though that is a low bar. One reason to think that is the case is that working donkeys' owners live more closely to, and are more dependent on, their animals, than operators of factory farms, meaning they benefit more from their animals being healthy and happy.
• Markets in donkey skin could have some pretty bad externalities, e.g., with people who rely on working donkeys for a living seeing their animals illegally poached. (On the other hand, this ban could also make such effects worse, by pushing the market underground.) Meanwhile, working donkeys do useful work, so they probably improve human welfare a bit. (I doubt donkey skin used for TCM improves human welfare.)
• On non-utilitarian views, you may place relatively more value on not killing animals, and/or relatively less value on reducing suffering. So if you give some weight to those views, that may be another reason to think this ban is net positive.