Alright, a hypothetical, since I am in a 223 state of mind recently.
So, probably not on the way out, but could it be that with new ammo (I believe US army has adopted 6.8 cartridge in the recent months), 223 is going to start being pushed down the list of staples and perhaps pushed up in price, while factories are going to start gearing up for the new latest and greatest?
I mean, sounds unrealistic, but! The two years we’ve had, nothing is unrealistic 
For now i think there are just too many 223 rifles out there for it to become obsolete, but history shows it will slowly fade away like the .222 did, like so many others did. I just dont think itll be very fast, mostly because of the after market AR scene in the US.
Here’s some more hypotheticals.
45-70 was a Gov’t. cartridge. Is it on the way out?
6.5x55 was a Gov’t. cartridge. Is it on the way out?
30-30 was not a service cartridge, but is it on the way out?
223/5.56 is a Gov’t. service cartridge. Is it on the way out. From the military, maybe. But civilian; I doubt it.
222 has been a very popular cartridge. Is it on the way out? Certainly getting harder to find ammo’ but still plenty of brass for reloading.
Even 303 British. Might be an historical military cartridge, but did it die? Not yet.
I suppose the definition of ‘dead’ is probably important than. I don’t think any cartridge is ‘dead’ in a sense of extinction, i.e. no longer produced. All of them are still made… PPU and Fiocchi make some weird stuff. So with that in mind, dead means low priority and low quantity. Using that definition…
I think:
45-70 was a Gov’t. cartridge. Is it on the way out?
Not dead, but ain’t that ‘alive’ either with $4 a shot for commercial ammo.
6.5x55 was a Gov’t. cartridge. Is it on the way out?
Probably very close to it.
30-30 was not a service cartridge, but is it on the way out?
Don’t know, certainly on the decline.
I think all you have to do is look at 30-06, an ancient cartridge surrounded by “better” performers yet here it stays and just as popular (and cheap) as ever. It’s a staple round that’ll be here until we stop making things that go bang and i think 223 is in the same boat.
Its a tough call to make. I could see the end for the 303 even though there are lots of gun in safes out there. They are rarely shot just because they are not as good as other cartridges or rifles. The 223 will be around for at least 2 generations like 50 years before people lose all sentiment with the AR / M16
FWIW, 6.8 SPC (Henceforth known as 6.8 Baked Beans or “6.8BB”) first got a serious look in for the US as a replacement for the 5.56 20 years ago. That’s 20 years since it was first tested for the US AF. I think 223 will be safe for a long time as even though 6.8BB has been available off the shelf (Aisle 3, next to the spaghetti) for that long plus a few years, its going to be a lot longer before the millions of civllian AR15s in the USA get fitted with 6.8 uppers, and unlikely that their 556 uppers will be disposed of.
EDIT: This is not the SPC and I am a retardo. This is 6.8x51
So is it just a necked down .308?
Never mind. Heres an article about it. SIG Ammunition Produced & Delivered Over 825,000 Rounds of 6.8x51mm Composite Case Ammo for US Army Next Generation Squad Weapons Program Prototype Test #2 - Soldier Systems Daily
Of the barrel…
Yep.
We have a comedian.
Probably…theres 223 and 556, same but different. Although, i still swear an Omark 556 i had was the most accurate rifle ive owned, im still not a huge fan…
.223 will be around for the foreseeable future. It’s become the standard for an intermediate cartridge.
It’s not going anywhere. It is the king of .22 centrefire.