One of the biggest challenges that we will encounter in Bitcoin is the Scalability problem which means there is only limited amount of information that can be contained or its capacity. I'm sure that there will be more people who will going to join the campaign which will result to a possible delay during processing or trading. So how can we provide a solution on this or how to ensure that all the trading done were saved once delayed before bitcoin achieves mainstream adoption?
If scalability won't be fixed then its not a problem since I'm quite positive that bitcoin will never be used as a currency. People who knew about the technology will never use bitcoin as a currency instead use FIAT or use another crypto to do transactions online that is more convenient. We do have lots of choices when doing online transactions that is why other modes of payments will coexist with cryptocurrency for more options if one fails to deliver.
Sooner or later there will be new cryptos that will address this problem of scalability so no need to worry about bitcoin's flaws for it will remain an investment vehicle and a store of value.