Yes realism. Realism is the central premise of airsoft. Realism does not mean milsim or having picture perfect guns or loadouts but it means having things which are largely based of real world counter parts. It is this aspect that makes a lot of people start airsoft over things like paintball. And I know airsoft guns don't shoot as far as real guns but they couldn't without being lethal at closer ranges but its the closest thing there is to the real world which people can do safely for enjoyment.
While I see you second point firstly it would be stupidly unworkable but also real world rifle would not necessarily fit those measurements which again would negate the point of having RIFs in the first place. Also for something like a DMR its not just length or fire rate but you also have to take into account magazine sizes and things like SR25s have larger magazines which mean usually one would not be able to carry as much as an SMG which again is an exchange for higher range and accuracy. Also here are lighter weight LMGs which have other disadvantages. For instance my M249 weighs a lot, but it is quite short. A MG36 on the other hand does not weigh as much but it significantly longer. It is this advantages vs disadvantages which makes airsoft so enjoyable and they do have to be controlled like FPS limits to keep the advantages there. As I've said before if all those went we would just be left with a really boring set of basic guns which people modified in LMGs or DMRs and snipers and things like HK417s or M60s probably wouldn't get made.
And that is not an LMG. As I'm sure you know that's a CBJ-MS PDW which does have a (albeit completely pointless) LSW version. Thats a light support weapon, not light machine gun. There is a key difference. For instance a L86 I would not give a drum mag because its not an LSW and the real ones don't use them. You also have to take into account things like barrel weight. While a AK74 and RPK may be very similar, just one slightly longer. In real life you couldn't just slap a drum mag on a AK74 and get it to function in the same way as the barrel in the RPK would e heavier and better designe to take a higher rate and volume of fire. So neither the CBJ-MS or the AKM would be classed as a LMG
I know this all rests on the idea of realism, which is a central basis of airsoft so cannot be simply ignored. While people are free to make what they want, don't be surprised if a sight owner said no to something like a MP5 DMR. I know if I ran a sight I personally wouldn't allow it and have heard of sites doing it before.
While you have been known to spend lots of effort into hiding battery cables, I on the other hand have taped batteries on the outside of my AEG for easier access. We're seeing airsoft from two very different perspectives, neither of us will make the other one agree and I suggest we leave it at that.
As for the CBJ-MS: The designer, unless I am much mistaken, will with great enthusiasm tell you how it can replace it can replace pretty much every roll, in different configurations. Then again, his views are far from objective. I do not have the knowledge to say either yey or ney. I suggest we leave this at that aswell.
I do agree with Sam about this being hugely off topic.
Maybe a moderator could split these posts to a separate thread?
Anyway, leaving it is what I will do, as I do not have the time or energy to get into this discussion, which I do not find very enjoyable.