I agree with the first part of your post completely but this last paragraph..... It's simply untrue.
First, you correctly pointed out earlier that ford is still making the FoST/FiST overseas; isn't that catering to enthusiast where there is a strong market for hot hatches?
Second, there are something like 5 different unique Mustang Variations out right now (Base Eco, Performance Eco, GT, Bullitt, GT500) not including convertible and/or performance pack options. Hell, just last week Ford had multiple enthusiast out to NorCal to review the new 2020 Ecoboost Performance Edition (RS motor in a Mustang). This included prominent Youtubers (not professional reviewers with a named platform) who are the very definition of enthusiast.
The truth is that hot hatches are a very small segment of the US market. I'm sure Ford ran the numbers and determined that the FiST/FoST/FoRS just weren't worth it economically. Yeah it sucks but it is what it is.
This is a weird way to look at it. You’re asking a market to exist for cars that have, historically, had spotty availability in the states. Halo cars, generally only exist when they have a solid following of regular cars. For the STs that would be the regular Focus and Fiesta respectively, but Ford has been killing its own market share for a while:
From 2013 to the demise of the gen3 focus, Ford sold around 800,000 foci. The biggest hit in sales was the YOY sales drop between the 2015 MY and the 2016 MY which is when people realized that Ford had not acknowledged the trans problems that were plaguing the car. Now I have no realistic way to know how many of those are ST models but Ford moved almost a half million Foci in 2013, the first 6 months of 2019, Ford has moved just over 12,000 of them. This isn’t a surprise, but Ford has seen pretty regular decline in sales and it isn’t linear with the disappearing small car market. Companies like Hyundai have seen a 50% growth in YOY sales with cars like the Veloster, something they attribute to the N model.
The Veloster, for all its quirks, is a well sorted car, the powertrains are solid, most everything works and it certainly hasn’t put owners in rental cars for extended periods of time, and yet with the introduction of a high performance halo model, they are seeing growth in a market that Ford wants to say isn’t worth it?
Also, FWIW there are 3 models of mustang, EB GT and Shelby. The only realistic difference is the engines available, otherwise your buying the same chassis/interior at respective price points. You can get an EB mustang with the same interior as a GT, it just depends how much you want to spend and which engine you want. EB premium and GT premium have the same options and interior, that makes them not unique with the exception of their engines. So saying there are 5 variations of the mustang, while accurate, is a bit gratuitous.