Maybe it's like anything - we don't recognize it or understand it until it happens to us. My therapist knows about it. She doesn't exactly call it MLC, this is how she describes it 'H has spent all his life pushing down his trauma/shame/shadow that it is now seeping out of the sides and bubbling up over the lid'. She sees it regularly in her therapy room. She nodded along when I mentioned the rising up of the shadow, but she frames things around trauma and what has to be buried to survive. How people with childhood trauma have to adapt early on at the expense of their own needs. So I think all of this is actually known in therapeutic and medical spheres. When we don't deal with our past trauma, it will likely start spurting out all over the place. Especially when we haven't the coping skills acquired through stable parenting. This is also a 'known' - part of our maturation can get stunted at the point of trauma. AND we can regress to the point of trauma under extreme duress.
IMO, the timing of it is because at midlife, we experience a lot of pressure - aging parents, children going to college, empty nest etc - add to that our middle years can be our most successful if we have a career. So, we are busy, and under pressure. Maybe yes, the reevaluation, the aging angst, but personally I think that is more part of a transition.
You are right, we are all right here on the forum, this is some sort of major psychological event. The duration of the crisis, I suggest, is due to how the person who experiences it manages it. Or not, as is often the case. It is a truism, that if we do the same things we get the same results. So, if someone is avoidant (which most MLCs are) then this is what they do - avoid. Only that is what led them to the crisis in the first place. Thus we can see why it takes time. How many times do we need to get hit on the head by a coconut before we move from under the tree that is giving us shade (this is a weird analogy, but it kinda works). Some people seem to add layer upon layer to their crisis. The damage keeps on growing and I guess that's why the refrain 'MLC gets worse before it gets better'. It gets better when they/we don't want to get hit with the coconut anymore.
I've come to understand my H's crisis through the lens of bipolar. It really maps onto that condition at the moment. The high of replay activity, followed by the low when it all folds. It is somewhat helpful, but ultimately, giving something a name only takes us so far. I think we can all agree, some people fracture under the weight of life, and that time is often midlife.