How could or should the 6.5 mm Grendel be improved?

[LRRPF52]

It appears to me that riflemen don't use drop charts, let alone have spotters with high-power scopes.

Exactly the point. In cases where riflemen have decent positions behind some type of cover, and are able to make well-aimed shots at enemy combatants where wind begins to be a significant factor, they will miss closer without any training in wind-reading and compensation. If they are able to see where their misses register, keeping in mind that enemy fire may be flying past their faces and impacting nearby, you could make the argument that a cartridge with less wind deflection would be of some benefit, but under combat conditions of 2-way gun-fighting, trying to see your hits is not exactly like it is on a 1-way range.

The "I'm hit!" video is actually atypical, especially when you dig into the back story. The soldier who filmed it with his helmet cam, was an LEO before joining the Army at an unusually old age, 38 IIRC. By his own account, he was attempting to draw fire away from his squad:

"I was heading down the face of the hill when we got hit. The rest of the squad was pinned down by machine gun fire. I didn’t start the video until a few minutes into the firefight for obvious reasons. I came out into the open to draw fire so my squad could get to safety. I was hit in the side of my helmet and my eye [protection] was shot off of my face.”

That's not exactly a battle drill that is taught or sanctioned, especially since he seems to have gone lower than them on the mountain, making it even more difficult for the unit to withdraw, as we never leave a fallen comrade to fall into the hands of the enemy. You are correct that indiscriminate fire is ineffective, regardless of caliber, but the "I'm hit!" video is a lesson more in tactics than anything. If one of his comrades with DM training was able to get into a good position without being seen, then place effective fire on the PKM that was lighting the "bait" up, this might have been successful.