I want to make it clear before I go to far into this I am more just clarifying some stuff than dismissing. I pretty much agree that we need optimal rest for best hypertrophy (or that with too short of rest you need to increase number of sets/volume to achieve the same level of stimulus) and that you probably don't need insane levels of volume (the 4-8 sets is a good reference), but that I don't really see how we can equate it being harder to maximally recruit fibers when doing higher sets (so my main point of clarification is that there is not specific reason to value these lower rep sets over higher set ones taken close enough to failure).
I think you may be conflating fiber recruitment with muscular tension (although maybe I'll touch on a bit later why maybe maximal tension isn't as clear or necessary as often presented). Much of the "effective reps" methodology is on BOTH needing to maximally recruit all muscle fibers WHILE exposing them to near maximal tension.
We know training close enough to failure with nearly any rep range will maximally recruit muscle fibers and we have evidence that if we train this way that hypertrophy gains are similar in lower and higher rep ranges.
Reasons to train at various ranges may come down more to lifter temperament and type of movement. It certainly is easier to gauge how close to failure you are when you aren't super high reps (varies but definitely under 12-15 reps) and training with high reps SUCKS (is very painful and hard to get to true failure before stopping due to issues of fatigue that are different at lower rep ranges).
"5 effective reps" from failure comes from the idea muscle recruitment is maximized at roughly 85%, but part of the problem with simplifying it too much is that there is research that more well trained lifters can maximally recruit the primary training muscles much sooner (and therefore may not need to train as close to failure to achieve a stimulus and may be able to be getting "effective reps" earlier in a set or at lower percentages of max). Accessory muscle groups do need tend to need to be closer to failure to be maximally recruited in compound movements though.
So overall the more well trained a lifter (or the better they are at recruiting a certain muscle and generating tension) may need to train further from failure than a less seasoned lifter.
I am just not sure that the bolded is actually shown in the research. I'm not entirely sure I am understanding what you are implying with not needing to do more than 8 reps (that seems like a pretty strong assumption based on the effective reps model, I don't think even the staunchest proponents of it argue anything like that). Effective reps seems to hinge more on equated hard sets than trying to specifically count all those individual reps and sets at higher ranges are still highly effective given proper conditions are met.
Fwiw not sure about the specifics you are alluding to with calcium ion build up interfering with muscular contractions. Peripheral fatigue (one of which is calcium ion buildup) tends to be less deleterious to performance than CNS fatigue, but even if we take it into account generally metabolic accumulation occurs more in concentric actions (builds and relieves quickly) and calcium ion accumulation is more an issue in eccentric motions (slower accumulation and relief), so it kind of depends on the movement and execution.
I definitely think mechanical tension is the primary driver (of at least maybe the initial stimulus), but think we shouldn't throw out other potential mechanisms. There is pretty interesting data that I have seen (both in studies and some interesting case studies I've worked with) showing potential benefits from muscle de-oxygenation and hypoxic conditions (you could think kind of like what you see with blood flow restriction). There are also things like studies showing lactate triggering MPS (and you don't want to get me started on lactate stuff that isn't related to this topic and how outdated people's views on it are

).
I apologize for the long post and I think the framework of effective reps "works", but mostly just because it gets people to make sure their work is hard and to use that to quantify sets being better probably than previous sets x reps (and similar methods) ideas. Just a little confused by the assertion that things over 8 reps don't have use cases or are less effective.