That's not quite right - the Devs said they considered full stacks ie. The china approach to all civs in game but abandoned that.
I think you just need some creativity to get an approach like this working. I could see something like a HOI4 tree for each civilization where you have branching paths representing different cultures within your overall civilization (including some alt history ones), and some civs could have common branches. I think that would be really fun, immersive and much more flexible than the current implementation
They already have 'branching paths' in the sense that no Civ has only one path to it, and every Civ has some path to it based on how you've played up to that point in that particular game rather than any 'fixed' progression.
Examples:
Chola can be chosen if you've played Khmer or Maurya in the Antiquity Age - the 'geographical' progressions, or are playing with the leader Ashoka. But it can also be chosen if you have 3 settlements with City Centers on a coastal (sea) tile - in other words, Chola is sea/naval oriented, so if your Civ is already 'on the sea' it is a possible progression.
Mexico can be chosen in Modern Age if you've already played Maya, Inca, Shawnee or Spain as Civs, or are playing leaders Isabella or Tecumseh - basically, all about the Native American and Spanish traditions that make up modern Mexican culture. Or you can play Mexico if you have at least 3 Distant Lands settlements in Desert or Tropical terrain - in other words, in-game you've colonized a Mexican-type terrain regardless of what 'Old World' culture you played.
These are by no means perfect: you'd be hard put to find any Shawnee influence on modern Mexican culture, but that's a factor of the relatively limited number of world-wide choices available so far in the game.
More importantly, I think by building on the in-game developments as 'requirements' for progressions, both flexibility and some kind of believability can be obtained for all or at least most of the choices. No matter how you slice it, there simply is no coherent Antiquity 'Civ' as a predecessor of any Viking Exploration choice, nor has the game yet provided any Central Asian predecessor for Russia, Mongolia, or anybody else north of India between Rome and China. In fact, again, there is no northern eastern European coherent 'Civ' as a pre-Russian possibility.
On the other hand, if one presumes that Viking/Norse = seafaring then having Antiquity settlements on the coast or X number of ships at the end of the Age could allow almost any Civ an in-game path to 'Vikings' - they might exchange their helmets for bamboo hats, but they'd still have some legitimacy in how they got to the longships-raiding-trading-over the ocean to steal you blind aspects we've come to know and love in the Vikings - or, if not 'love', at least Expect.