The main issue is aiswarya is not accepting what pakhi actually is. She is still in a denial mode and it is so evident in interviews. Even in last interview with Neil she was blaming sai and tried to make pakhi look bichari. Then it's Neil who finally said pakhi know virat can't be hers but she can't see him with anyone else.. That's her problem.
So first aiswarya need to accept pakhi as grey which soon turn to black. Then she may be able to portray character better.
And I don't think aishwarya would have performed Better if she was given sai role. First she doesn't look like 18 year old which ayesha looks. Second sai's character is not always bubbly. It has many layers including emotional one which I don't think aiswarya can pull off. Emotional scene ko chodo ... Just imagine sai kaku takkar scene with aiswarya as sai .. it would turn to physical fight ..
I'm not saying ayesha is perfect.. Actually in initial days aiswarya was better in acting terms than ayesha. But then ayesha improved tremendously especially in expressions.. She can make minute expression and can make it visible to viewers... where as aiswarya graph went down.