To be fair, there's very little separating the Yaris from the Fiesta and the Sonic and the XA and the Rio and the Accent, etc... in the eyes of the average consumer. Hell, the average consumer probably couldn't tell the difference between most of them. Yeah, the Fiesta won some awards and it has nimble handling, but considering the market there's nothing really special about it. Up to and including the SES, the Fiesta by and large is an appliance that had some initial hype and found a small selection of enthusiasts who find value in it. They're a good quality cheap car, which works great when they turn a profit...but they don't really do that very well, and a lot of salesmen consider them to be a waste of their time. Let's be realistic...most of the people buying standard model Fiestas aren't buying them because they're the cool car to buy. They're cheap and available...and in the cheap and available car market, the Fiesta has a LOT of competition.
Ford has been losing compact/sub-compact customers to Asian manufacturers in droves for the past decade and a half. Especially now that emissions restrictions are being rolled back, the incentive for American manufacturers to make subcompacts that compete with companies who are already winning the fight is diminishing rapidly. Point being, it stands to reason that Ford would drop the Fiesta. It sucks we'll lose out on future Fiesta ST's, because it's a car that really has something special to it...but outside of the Fiesta ST enthusiast community there aren't a lot of people who will notice a hole in the market.
Could they attempt to re-animate the car? Sure. They could try to recreate the hype from a decade or so ago, but that would require an expensive complete redesign of the car that would probably end up going the
exact same way the 2008 Focus was received. So...dropping it makes sense. It's not the last time we see the Fiesta in the US, I'm confident about that, but it will take a massive jump in oil prices, limitations on imported compacts, and dramatically increase in price to come back any time soon...so basically I'm guessing we'll see it re-introduced within the next 2-3 model years.
That all being said, as it bums me out to see the ST go away with it, I'm finding myself closer to the "Good riddance" mentality than "Goodbye my love!" It was getting stale, and I look forward to the next iteration of standard model Fiestas (whenever they get brought back) FAR more than I'll miss the current. Until then, I can't think of a better car on the market for me than the FiST I have now.