Isn't it fair to say that any currency can become collectible? Numismatics for example... what is the value of an ancient coin now versus when it was held as currency? So, if Bitcoin is seen as cool/valuable, people will collect regardless of its ability to function well as a currency.
I think it is possible to see bitcoin as a currency that is widely used in online payments. The development of bitcoin services in billing is proving the convenience of bitcoin in the transaction so I think bitcoin can completely replace the paper money in the future.