Robin, it definitely is, just as you indicate, a matter of both anatomy and dexterity when looked upon in an individual perspective and of course individual players (hopefully) choose instruments which suite them personally rather than desperately insist trying to learn an instrument which for some reason is unnecessarily awkward. Still it may be admirable in a way playing violin with one arm but I gave it up pretty soon myself when the right arm was paralysed from an injury...

When constructing/designing an instrument ( or whatever tool) meant to be used by a wider population you better try to consider a wide variation of anatomical data hoping to make it accessible for almost everyone. Talking anglos for a start the hand position vs the keyboard layout ( for the standard 30 key) *may* be almost ideal for the majority but for people with large (long fingers firstly) hands the hand strap comes over the knuckles and locks the hand, for people with short fingers a good position vs the handrest/strap means that the distant buttons can not be reached at all with the little finger unless you slide within the handstrap or constantly rotate the instrument ( clockwise looking at the right end) Many anglo players adapt to this playing position spontaneously but it is not particularly comfortable.

You may compare trad british style "anglos" with german style six-sided 20 key instruments. These from tradition have the keyboard located further to the top of the endplate and the hand bar is located at the geometrical centre of the endplate, The result from this is 1) you hold the instrument in a more balanced position 2) you reach the button rows with fingers in a more neutral position ( speaking about "ordinary people" with midsize hands.

Naturally these problems can NOT be solved in a simple way for all size hands unless there are means to adjust the position of the handbar/handstrap. ( Still talking anglos... and duets, which have the same arrangement)
The same of course with englishes. The position of the thumbstrap and little finger rest have to be adjustable to become suitable at all except for players with really small hands. The distance between the button rows shows that also. Is it not surprising that such evident needs have not been dealt with during 150 years??

The english concertina fingerplate remains being a total mystery. It WAS originally designed to be the resting place for BOT 3rd AND 4th finger BUT later on almost nobody plays like that but uses it for 4th finger only which of course makes both the design and the location of the trad fingerrest completely absurd....strange....