About “it doesn’t matter which hand you use”… I’m not sure if I understood well what you said, for the mouse it does not matter either. It is assumed that when you use the mosue with your left hand, the user changes the mouse configuration for left-handed user in the OS. So then it does not matter if a button is the left one or the right one, for each equivalent finger of each hand, then the same action corresponds. If you have configured LMB to select in Blender, then index finger of each hand is the one that will execute that action in the mouse (if you are using the mouse with configuration for left-handed user in your OS when you hold the mouse with your left hand, of course)
I say this because to rest my shoulders and hands I have become accustomed to using the mouse at times with the right hand or the left hand (I have a shortcut in Linux to make the configuration change quickly in OS).
The brain does not work first having to calculate which is the left button or which is the right one to then perform the action. Luckily the operation is more direct, if you learned to use the mouse with the right hand, then what each finger does (index finger, for example) is the action that will naturally make the corresponding finger of the left hand, without your Brain stops to think even for a moment about whether you are pressing right or left button.
Now, about Ton decision for RMB, I’m not sure if by that time it was possible to configure mouse for left-handed users from the OS