The idea of 'complacency' as a sense of smugness is only one part of its range of meanings, and in my view not its most up-to-date sense. Unfortunately the most recent quotation which the OED provides is from 1875 !
My own view is that the modern sense in which 'complacent' or 'complacency' is most commonly used does not centre on an idea of smugness. It rather focuses on a presumption that everything will turn out fine, when the likelihood is that it may not, and stresses an ignorance on the part of the complacent of potential pitfalls.
'The Manchester United board, in making a simple managerial change, were guilty of complacency that last season's success would be easily repeated'
The above does not necessarily mean the board were smug. It simply means that they thought the task in hand would be easier than it turned out to be.
In fairness to the OED, though it is clearly behind the times with its quotations, does provide as its first meaning, one in which 'smugness' (according to ODD 'having or showing excessive pride in oneself or one's achievements') is neither mentioned nor implied.
Etymology: formed as complacence n., with the later form of the suffix, -ency suffix.
- The fact or state of being pleased with a thing or person; tranquil pleasure or satisfaction in something or some one.