BUT if BTC cannot serve as storage of wealth for all and neither as day to day currency due to very slow and increasingly expensive transfers, what role will BTC play in the future?
I honestly think that it will become a billionaire's toy in the future and coins like XLM which offer incredibly fast and inexpensive transfers, will play the major role in the financial day to day transactions.
I think if bitcoin becomes really rare, expensive and for these reasons hard to acquire in the future there is a chance it will become a collectible token like those very valuable old coins or paper bills. On the other hand, the number of decimal houses can increase from eight to any higher number, so there will be fractions of bitcoins for everyone in the world willing to adopt it.