I guess people who have strong intention to purchase bitcoin for investment purpose will always find another way to buy bitcoin, even if Apple credit card does not allow their customers to purchase bitcoin, I still believe the growing potential of bitcoin will not be affected at all.
I also believe of growing potential of Bitcoin. Bitcoin dominance already played a role in the last 2 months, almost break the last one years or more records. I think Apple will not be able to play an effect on Bitcoin.