I agree that more tools to keep the peg would be good.
In the example you are taking, there are two aspects to consider:
- if the SBD is $2, then the real world transactions revert to fiat, as before the SBD, waiting indeed for the $1 peg to be re-established. Some people who used to pay with SBD will revert back to paying with fiat.
- on top of it, here intervenes the beauty of steem: there is still a certain probability that some people will "overpay" by paying with 1 SBD that the vendor (who gets the SBD) only takes as "face value", i.e. $1 if the following things hold: a) it is pretty easy to gain SBD by doing something you like doing, like for me writing AND b) it is pretty hard to convert those SBD into $2 of fiat, as is my case for instance because I don't have an account with Bittrex so I would need a full set of contorsions to extract maybe 15 extra bucks from as many SBD ... not worth the trouble
Generally yes, it would be better if witnesses had more tools to enforce the peg and if the peg was more stable. But I think you can trust the SBD to be "relatively pegged" - with a better certainty than for instance STEEM.
After that, it's a matter of how much risk one is ready to take - as I said, there is not perfect peg and a very shaky peg is still better than no peg at all.