2x what damage? The rolled damage? So you roll a 2 on your d10, and your greatsword's critical hit is a whopping 4 hit points?
With "Max damage + 1 extra die", you get several benefits:
- You get a significant amount of damage, beyond what you could roll in a regular attack, so the crit feels special.
- You take into account the weapon used. A crit with a dagger would deal 4 + 1d4 (5-8 damage), while a greatsword would deal 10 + 1d10 (11-21 damage).
- You get something to trade off for effects: for instance, you could have the ability to give up the extra damage and knock your opponent prone, or push it back 5 feet.