What Made Age Of Empires Great
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Win conditions are different from other popular RTS games. In Age of Empires, you can win peacefully, by building and defending a Wonder, like the Great Pyramid or the Colosseum, keeping it standing for 5-10 minutes, or by capturing relics, artifacts, and ruins and holding them for a set period of time. These win conditions have a lot in common with 4X games like Civilization, emphasizing more than just military dominance. It's not surprising that Age of Empires has a lot in common with Civilization, of course. Bruce Shelley, Civilization's co-designer, was the director on Age of Empires
Age of Empires has always leaned closer to the Civilization end of strategy games. While RTSes have gradually lost market share to MOBAs, 4X games like Civilization and Total War (which blends 4X and RTS mechanics) have thrived, and city builders like Banished and Cities: Skylines have sold millions. I believe Age works for this same reason: it's slow and sprawling, and lets you focus on more than just military engagements.
Modern RTSes have increasingly fallen prey to Grognard capture, focusing their attention on their most hardcore players, emphasizing high APM and streamlined economies. Focus is increasingly paid towards competitive play, ignoring the non-competitive players who are far more interested in the base-building, economy-managing, research-oriented players. Nearly every high-profile RTS is built with a heavy emphasis on this fast, competitive play style. There's nothing wrong with this kind of game--Company of Heroes, for instance, is fantastic--but without slower games like Age of Empires, a huge niche is being underserved.