It's a complicated issue. Banks are partly afraid of adopting something new. If Bank A is amongst the first American banks to switch and something goes wrong, Bank B may win its business due to customer frustration

I highly doubt that banks really care about "customer frustration." If they did, then they would be focused on fixing a million different existing problems.

I didn't say that banks cared about frustration. I said that banks would lose customers

Given that, despite all the customer frustration that exists right now, they haven't been losing customers. I don't think that Chip+PIN failing to work correctly at first would cause customers to switch. Chip+PIN cards would only be cycled into use gradually, as people replaced their older, swipe-only cards with Chip+PIN cards, or signed up for new accounts. There would be more than enough time to sort out any problems, and you could also start out by making Chip+PIN optional for new/replaced cards.

This is exactly what will happen. During the transition you will still be able to use mag swipe, then after some period the reader will force the use of the chip and only use mag swipe as a fallback when a chip error occurs. I suspect after a short while mag swipe will be removed entirely, but it remains in many places outside of the US as a fallback.