This has happened for the first premium skin, Unseen Elder Blood Armor, but there are two major issues with this:
1. [From the users] You actually have to play Unseen Elder, which you might not want to.
2. [From the devs] They need to put a lot of time into developing these skins, which only a small percentage of players can use.
The conclusion is that it might not be worth creating premium skins, without decoupled leaders. Now, with the decoupling, the devs can create premium skins per faction, which more players can use (and thus more players will buy).
Yes, I have only mentioned the premium skins because that's the only relevant aspect of the decision. As such, I have a new suggestion, which is sort of a middle ground (and more feasible): Re-couple all leader skins, except for premium skins. For example, Meve will always be Boosting, but you can use Radovid with every NR ability.