I honestly think that any able bodied person with the right level of dedication, time and instruction could achieve a first rate piano technique.
....the problem might be simply that that people who have professional-level mechanical skills had something in their physical makeup that allowed them to master mechanical difficulties relatively easily....
....many of us amateurs would simply never get a hard piece.... up to concert standard, however much we practise with whatever method, simply because we were not born with 'pianist's hands' (a shorthand for notions like natural suppleness, finger independence, neuromuscular coordination and no doubt other things).
Many long-term, dissatisfied amateurs like me have exposed themselves to various books and teachers, who ritually contradict each other, each teacher giving advice based on their own experience with what worked for them (with their pianist’s hands) plus commonsense thoughts about how their pupils (who may not have pianist’s hands) could practise to get the required skills.
Horowitz used flat fingers even in fast passages, unlike most others, but his finger work was as good as that of many other great pianists. In Schonberg's 'Great Pianists', we read that some good octave players used only their wrists, while others used larger movements. Some say these days that you can't play fast scales using thumb-under (=thumb turns), but is it really true that every fast scale player in history used thumb-over (=jumps instead of thumb turns)?
im somewhere between this belief and the first posters belief...i believe the vast majority of people with average co-ordination and physical ability can get a CONCERT-PIANIST-LEVEL-TECHNIQUE.but THIS IS NOT A FIRST RATE TECHNIQUE, let me make this clear.there is a big difference between the AVERAGE concert pianist's technique and the supreme elite.i believe the supreme elite go that extra mile in terms of effort and practice, and on top of that have 'super-natural' physical inclination, can any average person become a great runner? surebut can any person become the world record holder at the 100m race? no