Honestly, I'm not sure Bitcoin will still be relevant by then. I believe in Bitcoin now, but it's just that our world is changing extremely fast, and even in 50 years some radical change in technologies might happen that will make everything we use and value now (PCs, smartphones, the Internet, cryptos) obsolete. There's also a fairly good chance that 100 years from now we'll see a breakthrough in quantum computing, and this will make cryptography as we know it obsolete. Apart from that, tensions are rising in various regions of the world, not to mention the climate change crisis which is already starting, so perhaps in a 100 years people will be devastated by rising water levels, tons of hurricanes and tsunamis, relocations and wars, and in that world perhaps barter would be way more common than using any currencies. If all of this doesn't happen, and humanity stays relatively alright, I think that once everything's mined, it will only continue to rise in price.