There is one issue that it is hard for a fully autonomous blockchain agent to actually know when it is selling HBD for more than $1 (since it can't trade directly against centralized dollars and it can't get a true real time price feed). So in a fast moving market when the price feed is inaccurate, it would be selling for under $1 by mistake and losing money instead of making money. Of course investors could accept that risk when funding, and it probably wouldn't happen all that often.