I wanted the proposal to burn a significant amount without necessarily being overwhelming.
If people want to burn more, can easily make another proposal as well as this one. I didn't want to go with a huge number that could very quickly eliminate the DAO funds, because if this was pushed up above the return policy maliciously (for example because someone lowered the return policy in order to facilitate something else, and then this was pushed up in response, temporarily having a false kind of consensus), that any damage it would do would be limited by the time it was up.
The idea is to somewhat reduce Steem inflation when it is desirable, not to eliminate DAO funds entirely, asap.
Another way to do this would be to have multiple smaller proposals (say 100 SBD each, or perhaps an assortment of sizes like physical coins: 100,100,100,100,500,etc.). As many as desired could be voted up and the rest voted down.