Growing up in Sydney in the 1950s and 60s, I rarely heard 'so long as'. 'As long as' was much more common. However, these days the reverse seems to be the case. The phrase 'so long', an informal way of saying 'goodbye', was very often heard in those days, especially in American TV shows and movies. Is it possible that the increase in usage of 'so long as' is a sub-conscious extension of 'so long'?