Relevant to the discussion.
http://www.autonews.com/article/20161205/OEM04/161209933/ford-sees-lineup-changing-to-match-demand-will-add-debt
Ford will want to keep catering to the Fiesta ST demo for as long as they can. Companies can only offer what people will buy. With Fiat falling behind on the 500, and other manufacters, Hyundai/Kia for example possibly joining in, it would make sense to continue to continue to set the benchmark for the competition, by selling the only car this demo can/wants to buy.
But looking at current trends, even in Europe many of the Hot Hatches offer a auto/DSG or don't even offer manual at all, instead going for paddle shifters for that "sporty feel". These cars have more features than even the Euro-spec FiST and in turn are more expensive. The next-gen will add all these expected features for a price as the Fiesta moves upmarket. Of course Ford has not said if the ST will follow the mantra of "lightness, simplicity, and performance instead of creature comforts" that the demo wants. The basis of the ST line-up is to cater to that manual-only crowd, who also don't want features line lane-detection (many people shopping for the FiST attempted to find one with just the Recaros and ended up ordering them in this manner becasue dealers would not carry this, it was always the seats and nav/sunroof/wheels). In the US this crowd is dwindling, along with people who want new "premium/sport" small-cars at 20,000 plus dollars that don't appear like it could off-road. And even if these people cry on places like Jalopnik for one, will they even actually buy one when the time comes? For example, the Fiat 500 Abarth, completely falling out of favour, with certain trims offering 5,000+ discounts. Even between a used Fist/Fost people are steered to the Fost because it fits with the continued and still growing mindset that bigger/more power is better. Couple this with the fact that first-time buyers into the Ford family with the FiST may move up and that for many this is a 2nd or 3rd car, and the quick turn-over (many people have sold theirs for what-ever reason within 10,000 mi), US market control of oil/our dependence on domestic fuel, Trumps admins nationalistic rhetoric, the increasing population in city centers (the same demo Ford tries to get hooked on the smaller Fiesta) where these cars are a hassle, increasing cost of insuring a new car, as well as more traffic deaths/accidents, current trends and market history is not on the Fiesta STs side.
I would welcome a next-gen because it would mean the interest in such cars may grow, and in consequence interest in the younger-gen, more autocross, rallycross, aftermarket support, more focus on driver programs.
Though I do think that this gen will be known in NA as what started it all, and one of the most raw accesible drivers cars and would not be saddened if they quit imports of the ST, I've already got mine.