I think it is just a matter of "new marketplaces". Paypal benefited from the eBay marketplace ...and found casual online merchants.

Generally, marketplaces get hooked with very few mainstream payment methods.

Next time there will be a completly new marketplace, with new types of merchants and new market mechanic...you will see a new, major player...but it has to be disruptive to PayPal.

Online to Offline is a new marketplace in many ways...looking for a unified payment. If i am not mistaken, the next big payment player may come from that....