I think it will struggle for sales since it needs to be accompanied by an iPhone to be useful.
It reminds me of those 'mini credit cards' that were introduced a while back that seem to have faded away.
The reasoning was that they would be more convenient to carry as they are smaller
However they don't work in all machines
So they also come with a full size card for use in such cases
Which means now you have to carry 'two' cards
So why not just carry the large one?