Please, mister - 'ow can sumfink be 'almost certainly'? Innit like 'a bit pregnant'? You is or you isn't. HYPERION dominated for so long - I remember having an old b&w press photo of him and perhaps three others from his stud, titled 'A Million on the Hoof', since that was their value then. Now it's extremely difficult to get away from the influence of Le Grand Bonker: SADLER'S WELLS, who must've covered half the planet's TB mares in his long life. I suppose, provided he doesn't throw wrong 'uns, it'll be SEA THE STARS for the next two decades.
Haven't we reached a final destination with TBs, though? I can't think where they can be taken to next in terms of optimum skeletal health, respiration, etc. If we de-junk the badly-conformed, so that they don't pass on defects which are counter to best performance and are likely to cause arthritic and other conditions later, de-junk those with other known defects such as poor feet, tendency to bleed, sire wobblers, and so on, then I'd have thought we'd got the animal pretty much as far as it can go.
Speed is and will be determined by the skeleton, particularly the ability of the leg bones to take the weight of a heavily-muscled rear end, and the pelvic/hip/stifle to provide the driving mechanism.
Distance and endurance is and will be determined by heart and lung capacity, regardless of the animal's shape. KAUTO STAR shows that a relatively narrow animal isn't hampered, per se, provided it has the girth, shoulder line and is well-conformed behind.
Both types, and middle-distance specialists, too, still need an appropriate attitude or personality. It doesn't matter if you have a fizzy sprinter, provided its fizz isn't all spent in the parade ring or faffing about in the stalls. It doesn't matter if any horse has a plain head, scrawny neck, even a low-set tail (although it's often paired with sickle hocks), provided its underlying skeleton shows up in otherwise good conformation. Forget the straight-shouldered horse on upright pasterns - it's at best going to be a daisy-cutter, constrained by its poor angles, incapable of ever striding out. Going fast with short, sharp steps, yes, but that will only work on Good to Firm, and not over distance.
It's when ignorance or misguided experimentation mixes builds (or allows bad builds to breed) and functions that problems occur. Horses for courses, yes, but horses for duties, always. You must breed the right type, the right shape, the best temperament, for the job you expect it to do. (And while this might sound like stating the bleeding obvious, just take a look at how many get it very, very wrong!)