Blue
Ravenous Bugblatter Beast of Traal
I had mentioned that you needed about the same number of actions for attrition, and then modified it that actions with durations can be more effective so you need more actions total when you move to fewer encounters.I did see that, but I guess I would say you mentioned it without really addressing it. Also, fewer bigger encounters also involve attrition. It is not an either or scenario.
Again, the dividing line is forcing use of enough cantrips/low-efficieny actions to offset the higher efficiency actions.
If the fewer encounters-per-day scenario doesn't push the casters into using a non-trivial number of cantrips, it is "either/or". It doesn't matter if attrition uses up some of your spell slots in a day or all of them - it matters how much you push beyond it.
Now, attrition can still come in - casters can use some cantrips, but not to the point where it offsets the high-efficiency actions. That's fine, as long as some other days are the other way around. Mixing up days so that sometimes the casters do best and sometimes the at-wills do best is great DMing and normal variation.
To give another example, the first one I gave. A barbarian who has enough rages for every combat (including that some may go beyond 1 minute and require another rage) vs. a barbarian who has enough rages for only some of the combats. That's a strong example of an either/or situation.