What can I substitute for butter in Alfredo sauce?
This is 100% about flavor. Butter will give you this, EVOO will give you that, ...

Butter will give you a 'richer' taste, whatever that may mean for you.
Extra Virgin Olive Oil, generally speaking is far more powerful and may become overwhelming.
Any other oil will give you a different taste profile. 

Sunflower oil may be the most neutral and therefor the most appropriate for Alfredo sauce. Cannola oil may be another neutral oil, but I've never used it myself.

Generally speaking, an oil is an oil is a fat (for emulsifying).