Honestly, we have discussed so many different things that I'm not sure what you are trying to say. Correct me if I'm wrong, but I believe your main point is that due to weight, recoil, cost, and currently used tactics on the battlefield... it is a bad idea to replace the 5.56x45 with one of these cartridges. I agree with you on that.
I honestly don't know about the battlefield. I don't have that kind of experience but that is the gist of what I got from reading stuff on LF.net. It also seems to match my very limited personal experience as a civilian enthusiast.
For similar reasons (weight, recoil, and cost) you also feel that these rounds are inadequate compared to 5.56x45. I am tentative to agree with you on that.
I don't think they are inadequate. Some of them are clearly superior. The .300 blackout will always be better at close range in suppressed short barrels. The 6.5 Grendel will always out range the 5.56. The 6.8SPC with OTM bullets will be better than the 5.56 with M855 rounds. I just don't know that they are the best compromise, especially for a civilian. Military, maybe, but I find it odd that no one else seems to have tried it for any length of time.
I don't see why dropping a couple of magazines from your current set-up isn't a viable option. Some of these rounds offer better range, energy/terminal ballistics, and maintain the same capacity as the 5.56x45.
I did a comparison between my AK and my AR before I settled on the AR, and I have had the AK a lot longer. I tried a lot of different setups and for MY use. 6 mags of 5.56 worked out better than 4 AK mags for my imaginary end of the world scenarios. (Plus with that setup, it means I can shoot more after walking a mile to get to the "back 40") I understand that you dispute the energy/terminal ballistics advantage... but that still leaves the advantage of range. And, because the only thing you see is the increase in range vs the increase of cost (which I'm not concerned about), weight, and recoil. It isn't worth it. Fair enough *shrug*.
I don't dispute there is an energy advantage of heavier rounds with more aerodynamic bullets. With equal bullet construction, it will always do more damage and retain that capability further out. Now whether that is needed or not, and whether it compromises close range effectiveness, particularly from a military standpoint, I don't know. Maybe the combined 5.56 & 7.62 is a better compromise than a single round trying to do it all. If you are looking at this from a civilian stand point, why do you need more effective range? How far are you anticipating needing to shoot in defense?
I will address the rest of your points. But, I want to just get this much clear, because you are concerned with me talking about each point rather than "the big picture". So, I figured we should square this away first. And, then move on from there. I will address all of the points you presented, though.
I don't know that you need to. You seem to grasp my "big picture" argument pretty well. You don't have to agree. It won't hurt my feelings.