The BTC protocol will pay the miners though transaction fee. There's will always be a market for buyers & sellers.There will always be Centralized exchanges offering customers 5-6% APY on BTC deposits.
When BTC does hit 1m / holders of will be able to take 4-5% or 5k - 6k monthly and live of the interest.Also I heard that BTC will have smart contract coming future so lot of BtC will be locked up in Defi .I only have a small amount of bitcoin right now, but I plan to buy more in the future.