I'll try my best to answer your questions.

• The coin is not open sourced because TenX doesn't want their competitors to copy their sources and have the same features as they have. This is coming from their co-founder Julian but I hear that their comit network is going to be open-sourced.
• I never said that fiat is outdated or oversold. Cryptocurrency is people's currency and I think it has potential to grow. As for your question, that is looking at crypto as a store of value. I think TenX is trying to make crypto into a medium of exchange where you can spend crypto at a stable rate. Currently, Bitcoin serves as a store of value but I think this can change with stability and low volatility.
• Some credit cards do offer better cashback. But I think the incentive with TenX is rewards with tokens and cashback. You get cashback for spending and you get rewards for every transaction that happens throughout the globe for your token holdings. I don't know any banks that offer that.
• Decentralization is the key but we need to bridge the gap. TenX can use a open source to distribute cards and use them but which store would implement that? On the other hand, Visa and Mastercard already has millions of stores that accept and TenX can partner with them to connect crypto to fiat. Maybe in the future, we'll see mass adoption of open-sourced payment system but now is not the time.

I hope that answered your questions. If not, you can check out TenX site and ask the team.