Your example of burning into reserve to be able to lift a load shows the commercial pressure where companies of aircraft not up to the task under bid those that are capable to get the job, but then need to bend the rules to make it work
Hardly the case, you presume far too much. it was the only aircraft in the country that could do the task, providing due caution was applied.
You mentioned Two examples of burning into your reserve in that one post old fella . Some of us have contingency fuel on board to help deal with the unexpected .
You may have the payload capability to load extra over and above the legal minimums, in our GA case you didn't have the luxury, it was not unusual in our case that a flight planned to have ten enroute landings had to be cut short due fuel and make the 25 min flight back to home. Delays at intermediate stops could eat into fuel as shutdowns were not made, indeed in high wind conditions could not be made, landing limit being 55kt wind. Fuel was always uppermost in the mind, because water was the only landing option.
Not unlike the early trans pacific with the SP LAX - SY, you hoped to make it in one hop but the howgozit sometimes required a stop in Nadi.