This contradicts itself. First you say failure should always be a possibility (I completely agree!) but then you seem to criticize a type of adventure design that in fact makes failure a very real possibility.2, Failure is and should always be a possibility -don't put an obstacle/encounter in or call for a test unless you are prepared for it to fail or the party to be defeated. (It's a thing I see in a lot in adventures where there is, for example, a hidden door that calls for a check to be discovered but needs to be found for the adventure to progress)
Put another way, failure is and should always be a possibility whether on the individual-task scale or the whole-adventure scale.