Once again you are talking out of our #@!.
An even better explanation with actual physical evidence is that the energy requirement of your 5K pace exceeds what can be supplied aerobically, and by the finish line the inefficient anaerobic component has caused enough fatigue (through whatever mechanism you believe in today, lactate or no) that the pace is no longer sustainable. However if you slow down, you are again within your aerobic capacity, lactate levels stop rising, and work can continue albiet at a slower pace.
Lactate doesnt NEED to CAUSE fatigue for lactate threshhold to be meaningful. It is simply correlated; you Richard, like many know-nothing nimrods, are not capable of understanding the difference between causitive and correlative relationships.
Lactate is a reporter of the level of anaerobic metabolism that is occurring. Anaerobic metabolism is terribly inefficient compared to aerobic metabolism, and therefore not nearly as sustainable. It also causes byproducts like lactate to accumulate. Does lactate itself or H+ ions directly cause the fatigue? I don't know, and it doesn't matter. It could also be that such inefficiency burns through available fuel faster than it can be supplied back to muscle cells. Whatever the cause, the onset of high levels of anaerobic metabolism means that a slowdown is not far away. The faster you can go and be under lactate threshhold (ie avoiding high levels of anaerobic metabolism), the faster you will be.
Unlike your silly ideas (which are stolen anyways), this model takes the whole dynamic system of the human body into account -- fuel, cardio system, and yes even muscles. You choose to look at muscles and ignore the fact that they are connected to something else and depend on fuel and oxidant to function. It's like looking at a rocket and saying that its maximum high depends on the size of the nozzle and ignoring the amount of fuel or size of fuel pump it carries.