Zenimax are pretty inexperienced with the games industry. Yeah they have Bethesda but Zenimax themselves have shown utter incompetence in being able to manouvre the games industry. From suing Notch because they thought gamers would end up buying his game instead of Skyrim to holding Obsidian to a ridiculous bonus standard centred on metacritic they've always had more of a movie centric approach because they are a more generic media company at heart.

And as a company without gaming at the centre, i.e. as a company that doesn't get the industry, they will look at WoW and try to ape it in an attempt to achieve its success. They won't recognize the trends and patterns that define the industry they'll just take a bog standard business approach to it... and that approach says subscription model because that's the wet dream of ANY industry, let alone games.