I can imagine a future where bitcoin plays a role similar to what gold does now. This would not require much scaling - it would be about few, but high value transactions. But yeah - more probably it will be replaced by something else that would have less costs (i.e. POS, https://medium.com/@zby/proof-of-work-8d8265def194 ), and/or scale. This is the gamble in cryptos investing now - it is conceivable that in the long run some crypto will be worth much - but you don't know which one. Maybe Etherium, which showed it can change rapidly, will do this, but most probably this will be something new. Maybe EOS or Tezos - but they are too much hype machines - this does not fit good development.