You are right to think about long term future of Bitcoin especially with its limited supply. And after all 21 million Bitcoins are mined around 2140 miners will still exist and will keep network secure and processing transactions but their income will come from transaction fees instead of newly created Bitcoin.

These fees could become higher and new technologies like Lightning Network might help keep them manageable. Bitcoin price will continue to go up and down as it is doing at this time influenced by global economics and how widely it is used but daily trading should still be possible driven by its fixed supply and ongoing demand as valuable asset.