2004, 2006, and 2007 for Dawn of War, CoH, and SC respectively. More well known titles like Red Alert 2, Command and Conquer Generals, and Warcraft 3 were even earlier than that. Starcraft 2 was the last "AAA" RTS game that I can think of. Deserts of Kharak is probably the only RTS game I've played in the last 8 years that I really enjoyed. I tried CoH 2, but the micro transactions put me off. Eugen's Wargame series and Steel Divison had interesting and deep nuanced mechanics, but single player was very rough and the multiplayer player base wasn't big enough to sustain a good matchmaking experience.
Personally, I think the main culprit is that MOBAs like DotA 2 and League of Legends are too similar in genre and won over much of the RTS player base. Anecdotally, most of the people I knew that were into SC 2 started playing League or Dota 2.
Edit: I totally blanked out on the Total War franchise. That's definitely a staple of mine, and still playing Rome 1 and Medieval 2 mods to this day.
Personally, I think the main culprit is that MOBAs like DotA 2 and League of Legends are too similar in genre and won over much of the RTS player base. Anecdotally, most of the people I knew that were into SC 2 started playing League or Dota 2.
Edit: I totally blanked out on the Total War franchise. That's definitely a staple of mine, and still playing Rome 1 and Medieval 2 mods to this day.