The content of a civilization of AoE3 is obviously larger, more complex and richer than that of AoE2. There is a significant difference in the cost of developing and learning a civilization between the two games. AoE3 does not need and is not suitable to have such a large number of civilizations as AoE2. Personally, 30 is the max.
In addition, AoE3 also has many better mechanics other than complete civilization to supplement the representatives of many historical cultures, such as cards, minor civs, mercenaries, revolutions, etc. It can only fill the deficiencies by the development of totally new civs in AoE2, but there is no need to work so much in AoE3 . For both the development team and the community I think that it is the most appropriate way to make the kingdoms or empires that were not powerful enough but still special in the era to appear in the game as mercenaries. In fact, there are successful precedents for doing so.
Anyway, it should not be assumed that how AoE2 develops should AoE3 do the same. We should treat them as different games. Splitting umbrella civilizations is a trend in AOE2, not AoE3.