I’m actually a big fan of the concept of the affix and it actually accomplishes the goals I assume that the developers wanted it to accomplish. There is a level of complexity with the affix in regards to planning pulls to exact percentages and another layer in positioning where the mobs physically die.
All of the above said, I think that the 100% spawn is completely unnecessary. If you plan poorly you get punished pretty badly by having another wave, while planning correctly skips it entirely. What this does is change pull patterns to the point where you exclusively want to finish the last boss before finishing trash, which I don’t think is particularly good design because it’s entirely predictable behavior.