We cannot even predict what bitcoin is going to do tomorrow so it is difficult to imagine that we will be able to predict what it is going to do or happen in more than 100 years in the future, however I think it is fair to think that miners will have to depend on the fees they get for their services and not from the block reward way sooner than that, is difficult to know when that will take place but I suppose that process will have to begin to take form during the next decade as the halving keeps reducing the reward for each block.
The future of bitcoin cant be predicted by anyone and what we can say is some speculation about it. The level of difficulty faced by miners who will continue to grow every bitcoin reaches 210,000 blocks is a sure thing. However, miners will still exist even if they have to depend on costs.
I imagine bitcoin will continue to get good adoption from various countries and many countries will adopt it as a payment system as mentioned in the bitcoin innovation.
As an asset that has a function as a currency, bitcoin can be superior to fiat in terms of transactions and simplicity. The community will continue to make it a multi-use currency that can be transacted for various purposes, especially we already have a flash network.