Should Manchu decisions be tied to culture?

[Holland989]

I recently played as Korchin (the pink thing in the Manchu-area) which has Chahar (Altaic group) as primary culture. Since my aim was to unite all the Manchurian tribes and eventually form Qing, I decided to conquer them and culture-switch to Manchurian. The moment I switched I found out that I couldn't form Manchuria whatsoever.
I looked up the decision file which says:

manchu_nation_1 = {
major = yes
potential = {
NOT = { exists = MCH }
OR = {
tag = MJZ
tag = MHX
tag = MYR

So the decisions are tied to nations rather than a culture.
It seems more logical that a nation with Manchurian culture has access to these decisions rather than three set nations IMO.

Opinions are welcome.