Ooooh .300 BLK
I have a very love/hate relationship with this cartridge let me detail:
Loves:
1) It is a triumph of open standards - AAC/Rem SAAMI'ed this cartridge as soon as they could, and wiped out the .300 Whisper (death to proprietary cartridges!
2) It's .30 cal, which means it has a very wide selection of bullets - But so does my .308
3) It's of a bore diameter that's legal to hunt and allows you to do so with a short carbine
4) Suppressed - this is becoming less an issue as suppressors get better
Hates:
1) Fanboys
2) Ballistics
3) Cost
In all, outside the hunting, or some military use case, I don't think the .300BLK is a wise investment, it's ballistics are not great, and in most cases are about the same as a heavy .357 bullet. Stability becomes a major issue with the subsonic rounds and very aggressive twists are used to overcome this. As such, I don't think I've ever been able to get what I would consider as a rifle shooter an acceptable group at 100 yards with subsonic .300BLK.
I won't tell you not to buy .300BLK, if you like it, and you have a niche use case for it, go ahead. However from a generalist perspective, the 5.56 can and does beat the .300, and other cartridges like 6.5 grendel, 6.8 SPC, or 6.5x45 match or exceed the ballistics, of the 5.56 and leave .300 in the dust. One person brought up magpul's lack of magazine support for these cartridges, to me this is not an issue, I have pmags, I use pmags, but I prefer plain-jane USGI mags (with the magpul follower), ASC makes aluminum/SS mags for 6.5 and 6.8, and the 6.5x45 works with USGI mags.
I built a .300BLK at the height of it's coolness, I shot 3 boxes of ammo through it, and put the upper back in the safe. I've shot it a few times as a test gun, that's it. It's just not that cool.
Now, to answer the question: "Is it here to stay?" My answer is an unequivocal yes, it's not going anywhere. Being an open standard a lot of people make parts for the gun, the cartridges can be made from cut-down .223 brass. Like the .30 cal bullet, it's not going anywhere. It may become more niche, but from what I've seen, the days of the "OMG SUPPRESSED SUBSONIC CARTRIDGE" are fading. If you look at what the new hotness is in AR platforms: .224 valkyrie trying to push a heavy 5.56 bullet to varmint velocities. Again, I don't think this is going to be the forever hotness, and I don't think .224 valkyrie is going to stick around, there are already other cartridges that do it's job better.
In all, I think the next evolution is going to be more mid caliber offerings in the AR platform (i.e. something bullet-diameter wise between .224 and .308) some stuff has been done with 6 and 6.5, one company (sharps) launched it's .25 caliber offering a few years ago. However it suffers from a very serious and critical flaw: bullet selection.
One of the major issues that exists with the .223 wildcat cartridge development is very few developers are willing to change the geometry of the case much, In order to get those longer, heavier bullets into that cartridge, they need to push the mouth back 1-2mm, and maybe the shoulder as well. Grendel started from a different place, and I very much commend Bill Alexander for thinking outside the box on it, but it will probably never ever see the same wide application as 5.56 even though it is a dramatically better cartridge, because no one wants to go out and and not only buy a new gun, but a pile of new mags to go with it. This is why the .300BLK is so popular, it's just a barrel swap, and has lead to a whole new market for quick-change barrel conversions for the AR.
Anyways, I'm going to quit before I get too ranty. If you like .300BLK and want to use it because you have a safe full of 5.56 rifles and are feeling a little bored, rock on. If you're new to shooting and want to get your first black rifle, unless you have a lot of friends who all shoot .300BLK, make sure it's a 5.56.
Last note: If you do have a .300BLK and a 5.56 be exceptionally sure you NEVER EVER EVER mix up the ammo. I have seen two people jack .300BLK into 5.56 guns, which resulted in a complete explosive disassembly of the firearm, I actually have the barrel from one of the guns I've been meaning to do a cut-away of.