I dunno. I could see Ford discontinuing the hatch Fiesta in a few years because of the price limiting sales, but it's not priced much higher than a Fit or (I imagine) a comparably-equipped Yaris or Versa. Plus the amount of support Ford is giving the hatch (see proportion of hatch press shots vs. sedan, and the overwhelming majority of hatches at the LA show) makes me think that even if sales are lower than they're projecting, it'll be here to stay.
The Focus may get near-as-good gas mileage but mark my words, it'll be more expensive. The sedan will probably start no lower than $15,000; I wouldn't be surprised if it's closer to $16k. And if they follow the Mazda3 model (which they have with the Fiesta so they will with the Focus), the hatch will start at $18-19,000, so there'll be plenty of room for the Fiesta.
Plus, I bet there'll be a gas price spike coming out of the recession, just in time for the Fiesta to go on sale. Even if we're not back up to $4-5, we'll be edging in on $4 gas and the Fiesta will benefit greatly from that.