I frequently visit the Steam Charts website to see how many people are playing certain games since it gives me a pretty good idea of which games are still popular and which games have a declining player base. I looked up ArmA 3 and I found these stats:

all time peak players on Steam: 56,185

players in game 20 minutes ago: 9,072

peak players within last 24 hours: 13,502



These stats are pretty impressive considering the game is now 9 years old.

Yeah, it is. I think those Life and ALIVE servers are keeping it going. Those are pretty popular. I haven't tried them myself though.

More than anything else the modding community has kept ArmA 3 alive. Without that the game would have died a long time ago.