When that happens (or when mining a block is almost impossible) the price of bitcoin will spike as the only way to get btc will from person who own some already.Miners will only be able to make money from transaction fees,verifying transactions
The major change will be in the way mining operations get done. The fees might just boom a lot because miners will have only one way of getting paid and that is mining fees. Because bitcoins will be mined fully and thus there will be no block that will be remained to add into the blockchain. However, whiner a transaction will be done then there will be need of miners who will just algorithmically solve the transaction and add the blocks.
On investors perspective, the demand will be very much high as whole bitcoins will be in circulation. The supply will be fulfilled already and only one thing will surge and that is demand thus rising the prices high.
For me if happens bitcoin will hit 21M supply and circulated the demand of the bitcoin in the market getting high and that's the time the value of the bitcoin will going up. We know that 21M supply of the bitcoin will not hit for now but hopefully it will be happen and I see the people here are still waiting for the best value of the bitcoin.