It is pretty easy to see Blizzard wants to keep upgrade tokens to drop from the relevant content difficulty.
This cannot be done with 1 currency.
Valor farming was degen gameplay, some 3k score people going into +2 to +5 keys doing them in 5 minutes.
Now people will actually need to do the content that they want the upgrade for.
This will also have a positive effect on group making for M+ imo.
Now, very high geared people won't join your +11 to +15 groups because they have literally nothing to gain from it (previously it was valor).
So people cannot just decline others for centuries until they find a farming 4-stack to carry them.