The request is for people who don't want non-boosted factories to not have to research those goods. So they'd need to provide a tree with marble, crystal, elixir required with everything else optional. And another tree with marble, crystal, dust required and everything else optional. And a tree with steel, silk, elixir required and everything else optional, and the others of the 3x3x3 possible combinations of cities (minus the one or two that aren't possible). And then because the trees for elves and humans are different, they'd need to double that. So every possible combination of city boosts for each race would need their own tech graphic and then code would have to keep track of which associated path to allow. Currently, the code just doesn't let you past that point in the tree unless you've learned all the factories.
Aside from which, that was without the inclusion of sentient goods, which means to get into the new chapter, you're going to need previously non-boosted factories for the new T4 (and then T5 and T6) boosts, anyway, so the whole discussion is probably irrelevant.