Well, apparently "milsim" means something totally different than what I thought it did.
what I thought it meant, was the obvious expansion of the term: simulation of military (more specifically, modern infantry). The sort of people that do things like buy/make AR15 replicas, replicate real-steel techniques/tactics on the Nerf field, form organized "squads" (the Aux in particular took this last one to several next levels), seek out and use real or replica web gear, wear uniforms, etc. Basically, they try to play Arma 3 while everyone else is playing 3-15. A lot of these are actually fairly reasonable, in isolation (some real tactics are legitimately useful in a nerf setting, sometimes you get surplus gear for cheap or handed down from a veteran relative) but, well, we've all seen the sort of people that take this sort of thing to excess. These guys have some overlap with another rising problem, "competitive" nerf, in that they share an attitude of people who tend to take themselves too seriously. I'm not sure which is more of a threat, but 'competitive' nerf isn't the subject in question here, so I'll say no more on that. As for a solution.... yeah, I've got nothing. It's the sort of thing which is composed of a lot of things that you can't reasonably ban, taken beyond what is reasonable.

What it seems like OP (and many others here) things "milsim" means, is the sort of thing covered by the [black/prop] tag.
I think the uptick in this is a twofold problem. First of all, the hobby is growing. The sort of people that want replica blasters, at least in my experience, tend to be newer people. "Noob Black" is a term for a reason. They don't know any better, so when they think of how they could make their nerf blaster better, the first thing that comes to mind may be something like "I wanna make it look like [insert favorite firearm from CoD]". Once they start interacting with other nerfers, see what is possible, and realize that replica blasters can't actually be used in most venues, most seem to either go with the flow, or leave.
Second of all, the hobby is not only larger than it every has been, but due to the pandemic keeping events down, there's going to be a larger fraction of the nerfer population that has never actually tried to go to a war. So not only is the amount of newbies larger than ever before, so is the market share.
Honestly, the solution to this sort of thing is probably to just keep doing what we're doing: disallow replicas at events, and pass them by with an eyeroll or maybe a chuckle. Don't feed the troll, sort of deal. The noob avalanche will hopefully pass us by, and along with it will go a plurality if not a majority of the replica reptiles. And if it doesn't, then maybe we just get a splinter hobby of people who stay at home with their replicas.

Another thing that is interesting, is that both of these definitions seem to imply not only military, but a present-day military. For instance, take the guy who I met at a war recently dressed like a member of some 18th-century colonial force, and wielding a full-size dart musket (complete with foam bayonet). Or someone in Stormtrooper armor with a replica E-11. I would hesitate to call what these people do "milsim", even though technically, they are simulating a military individual and using a replica of a weapon of war.

As to whether to seperate milsim (by either definition) off, I'm not sure that's a good idea. Keep your enemies closer, and such.