There is no agreement on what is sensible and what is not, the defaults are no more or less sensible than any other suggestion.
C-b is terrible for your wrist, especially in conjunction with the following commands.
Choosing C-a isn't as much about preference as it is about not-terrible ergonomics. Now people are right to prefer anything other than C-a or C-b but C-a is objectively a much saner default than C-b.
[0] http://xahlee.info/kbd/keyboard_ctrl_vs_capslock_position.ht...
I always use the modifier key on the opposite side of the keyboard to avoid that issue. So for ctrl-b , I press the ctrl key on the right side of the keyboard worth my right pinky and press the b key with my left index finger.