The better the textures, the higher the requirements, the less players.

In F2P games, it's actually easier. The people that keep the game going are the RICH people that have high-end PC's anyway. They won't suddenly be left out. In a largely subscription-dependent game like this however, you increase the system requirements and you instantly lose a noticeable chunk of your income.

People simply won't invest in a new, powerful computer just to play a single game, over paying their bills, buying food and stuff. And all of that while still having to pay the subscription every month, which in some areas of the world is a much bigger chunk of their monthly budget than in others.

Unless it would be absolutely optional and not affect people that choose not to use it at all (basically, requiring double the work for EVERYTHING that was ever made or will ever be made), the only upgrades are likely to be touch-ups gotten from optimalization...or on clients side.