Not enough people asked, and the devs have remained silent. I’m afraid we’ll have to sus this out ourselves.
Of the Age series, the only Definitive Edition to receive a full intro remake is AoE3, alongside all its cinematics.
(AoE1 gets the cover art converted into a stained glass; AoE2 gets a Premier slideshow of hand-drawn art.)
If you watch the AoE3DE intro yourself, you’ll notice that it’s a montage of many “snapshot” clips from scenes across time and spaces. It makes use of many character models, but they mostly perform short, confined movements, with few character-to-character interactions.
Also, that intro makes heavy use of high-polygon models made for the game (these models would be converted into the actual low-poly models you see in-game, and their details “baked” into textures):
They are more detailed than the original intro, but not extremely detailed, which saves the expensive cost of making unique models only seen once in the intro.
To our most benevolent devs, this state fits their positioning of AoE3DE, as a “side” game in the stable, with cinematics that are slightly “retro-looking” (by their official statement), that they don’t put at the front of Age series marketing.
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Now compare to Age of Mythology:
1. The intro requires unique models for the temple mural & statue scenes;
2. It involves actual action scenes of massive armies fighting.
These two factors determine that the intro would be significantly more labor-intensive (=expensive) to remake.
3. AoM Retold is also their headline product in the marketing for 2024 to 2025.
—It was shown at the Xbox press events, the only Age game to be shown beside AoE4. No other DEs received this treatment.
This determines that they can’t make the intro “retro-looking”, or use an upscaled version of the old intro. If they were to remake it, it would have to be at high detail levels that meet the current expectation for cutting edge CGI, which would multiply its costs by several orders of magnitudes.
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With this comparison, they have a good reason to not remake the intro. And their choice has paid off, judging from how few people cared.