Once 21 million Bitcoins are produced, miners' block rewards will only consist of transaction fees.

Once Bitcoin reaches the 21 million units limit, demand for the Bitcoin network may increase due to limited supply. This could cause the price of Bitcoin to increase. On the other hand, transaction fees may also increase, as block rewards will consist of transaction fees. This could make it more expensive for Bitcoin users for lower-amount transactions.

However, it is difficult to make a clear prediction in this regard, given that there is much more time before the 21 million Bitcoin cap is reached. Also, the Bitcoin protocol could be updated to address scalability issues as needed to respond to this situation.