In my opinion, thumb pressing sucks. The angle of your thumb compared to your other fingers against the buttons is just odd. Never felt a compulsion and many UK customers have questioned the common act of 7-button layout creation favoured by many American gamers. In my experience, it's just not something we ever did here and just feels wrong; it's not like holding a joypad.
With regards to the lefty/righty layout issue, as far as I'm concerned it is not an issue. Being a stroke survivor, I did a lot of study on the functions of the brain and the way it deals with body movement. Being a righty who lost control of his left-hand side, I came to this conclusion in one of my many restless nights in hospital:
I believe the original intention was that when two hands are used, the more favourable hand was available to be able to rapidly press the 'fire' buttons as the fingers on the right hand respond better for the majority (right-handed) individuals, both more rapidly and better for longevity (both essential for critical functions like jumping in DK, or long-lasting firing in vertical shooters), whilst the left hand responds better with less refined movements that do not need fine motor control (the fingers) but rather the whole hand.
Over the years I've very slowly recovered the use of my left side, thanks in part to perseverance and video games. My physios basically confirmed what I said above about the difference between large-group motor control and fine muscle motor control, so I tested the supposition whilst practicing during recovery. My left hand moves OK now and is adequate for general control of a joystick, but finger control is poor. Had I been a lefty, the fine finger control (which develops last in recovery) would have returned much better much sooner. I did witness a bit of 'hand swapping' by lefties back in the day, but only on fighting games where fine-finger control is more important (for executing rolling movements for special moves etc.
Thankfully I am a righty and am used to the 'correct' control layout of stick left/buttons right so fine motor control is not as important for my left hand. I will never be the combo-busting KI/Street Fighter master I used to be but I can play most things fine; if I'd been a lefty and had a 'reverse' layout, I'd be ---fouled up beyond all recognition--- cos my fingers wouldn't be fast enough.