Why are march fitness, agility fitness and weight carried such a evergreen topic?
Isn't the answer simple enough?

Self-discipline in training and in defining the loads.

It's really, really simple to trace almost all failures ever associated with these topics to failure in regard to the aforementioned requirement.

It's usually a failure of leadership either in regard to the leader's self-discipline during routine tasks or in regard to his self-discipline in making decisions. It's easy to dodge a difficult decision (weighing the pro and cons of loads) by just ordering the men to carry (too) much. In the end, the leader can claim that his mission was impossible because the men were not capable enough. That's an especially easy pretext when the leader hasn't been in charge (or the team hasn't been coherent) long enough to coin the fitness and competence of his men himself.

It's really not so important how much certain armies marched in certain ages per day. Leaders need to make difficult decisions and need to prepare their men, and it's always a trade-off.


What's interesting is not what others did or do; it's what kind of fitness your troops are expected to have (expected by higher HQ) and how you can match this and other expectations through exercise and load definition.

It's always a trade-off, an optimisation - and the cure-all for the problem is to arrange leadership dynamics in a way that does not encourage an overemphasis of loads carried and does not tolerate major training inefficiencies.
The solution is thus in the (junior) officer corps, not in weight tables, thinner fabrics or polymer cartridge cases. The senior leadership only needs to grow some political backbone and adjust its casualty aversion in a way that allows for strategic success.