The idea would chase the most active players away from the game. It is why no major MMORPG with a subscription works it like this. It would not be a good business model to essentially charge per hour to play a game.
Edit: Even if it were "optional" it would mean less revenue for Zos from the more casual players. Another reason why no major MMORPG with a subscription offers such an option.
It is a great idea for the player that does not play much, but not good for business.
Hm. With the current litigation happening about loot boxes violating State gambling laws (Google it, if you want to read up on this) may discourage changes to subscription, IF they have to change or lose revenue from crown crate random loot from them, since they qualify as loot boxes by legal description of the current court cases.
It's a no mostly cause the system this way would be designed to literally punish the people who play ZOS's game the most. That's the major issue.
Doing both this system and the old one would get frustrating from a CS point of view as people become confused on the differences and just opt in for the easy-to-understand 30 day sub anyhow.