After a long time of thinking, reading and also investing a bit into Bitcoin I have to say that it looks for me that the main use is actually just pure Investment.
Yes, I agree. It is 'digital gold', not a replacement for cash and cards. The current bitcoin technology is unsuitable for use as a payment system. The transactions are very slow and expensive, and the performance is severely limited by the protocol. Unless a higher-level solution is introduced, it will be more and more of an investment instrument.