The 12 year bit came in due to manufacturers concerns re internal corrosion considerations, remember hollow crankshafts were also subject to a calendar check.
TBO considerations were easier to contend with for engines in regular use, and sometimes also in line with oil sampling.
However there is no 'right' to an extension as it is up to the engineer in question and very much an individual issue based on knowledge of actual engine if he is the CAMO. If you change CAMO there is no guarantee of further extensions. In Holland they had a comprehensive cardex system with maintenance that laid out what needed to be done to continue engine use above TBO. Oil sampling and regular logged engines runs were also required.