I'm old enough to remember the flack from Sir Alec browning up for his passage.
It's the 21st Century, India is full of Sikh actors who are only a plane trip away (I dare say America and UK now have their fair share of homegrown Sikh talent).
Sikhism isn't just a religion.
It's almost impossible to convert to Sikhism so chances are you are going to look the part if you are fit to play it.
If they cast Benedict as another Botany Bay crew member they could have avoided some of this.