I know a lot of people subscribe to this theory, but I don't. Once again it's something that doesn't work in my brain. Going with this theory, for me, would invalidate one of the main reasons that Angel actually leaves Buffy in the first place. It also invalidates IWRY. If there is no longer a threat of him losing his soul, then there really is no reason for them not to continue being together after IWRY. Or even getting together later on down the road.There is a third solution that he will never experience true happiness because he's aware of the consequences. For him that shadow is always going to be there when he experienced it before it wasn't.
It's just not believable to me that one can control the amount of happiness that the person they are deeply in love with might cause them to feel, regardless of past experiences. Therefore, if I'm to believe that Angel was deeply in love with Cordy, then the threat of him losing his soul is still very much there for me.