I think that a change in caliber will require a redesign of platform, and any platform designed to unseat the M4/M16 FOW will have to bring more to the table than simply being “better”, refer to the SCAR-L. Any “new” caliber will be heavily resisted due to the financial impact of requiring LC to re-tool, as well as the NATO commonality requirement and the cost that would be associated with getting all other NATO countries to follow us into the rabbit hole, as we forced them to do first with 7.62, and later with 5.56, both with the promise that they were the best choice. I see widespread adoption of 7.62 to be more likely than adoption of 6.8/6.5/7mm Murray, regardless of their superiority in any particular endeavor.
FWIW- it isn’t just SOCOM that is using 7.62 ARs. The KAC SR-25 has been exposing the innards of jihadis for many years. It is NOT a 7.62 M16, it just looks like one. The Brits adopted a 7.62 LMT not too long ago, and it too has been fighting the good fight, so it isn’t just in “limited” numbers.
The only reason that the M16 FOW has the ability to be easily customized is because lots of people own them and dump money into the industry. 1913 rails aren’t all that mysterious, and they can be mounted on anything from M240Bs to coffee mugs. Replacing HGs on an M4 requires knoiwledge, experience, and tools to do right. There are several weapon designs that are comparable in furniture swaps. If the Mini-14 had the popularity of the AR we would see just as much modularity in furniture and optics.
M9 vs 1911? If you want to go there, feel free to start a new thread in “Handguns”, but for military application you are wrong.
But on your point about personilization: the 1911 does not lend itself well to individual user customization. Gunsmiths do it, for $$. Sure, you can change your grips, but past that and you need to do fitting, which is not something you pick up from youtube in 15 min. Sights, sear, trigger, barrel, slide, hammer (and more) all require fitting to work right.
M9? Pull the part out of the bag and drop it in.
Broken locking block? Pull the part out of the bag and drop it in.
Broken safety? Pull the part out of the bag and drop it in.
Cracked slide? Pull the part out of the bag and slide it on.
Cracked barrel? I think you see where I am going with this.
M2 and AK?
M2? Heavy machineguns don’t need much customization to do their job. There’s a reason that they come with tracers.
The only significant changes to them is a QC barrel, safety, and some optic mounts.
Still, there is an improved FN version that has been gaining ground and will probably wind up in ground units soon.
AK? There are plenty of accessories on the market, but AK enthusiasts tend to fall into a different spectrum than AR enthusiasts. AKs are generally persued as a cost-effective solution based primarily in robustness/durability with an affinity for alternate calibers. Even then, there are a few different mounting solutions for RDSs, AR-like stock adaptors, railed foregrips, improved trigger groups, and polymer magazines. There are a higher percentage of chinese craptastic parts being frequently added to AKs, but that’s probably just a result of the market base and audience targeting.
Everything evolves according to use.