Once all bitcoins are mined, no new supply will enter the market and availability will depend on people trading existing bitcoin. This scarcity could increase demand and pushes prices higher but it might also make bitcoin less practical for smaller transactions if prices become too high.
for all bitcoin to be completely mined to anywhere close to the 21 bitcoin that is in circulation, it means that all bitcoin holders won't be ready to trade bitcoin at all but will prefer to hold and leave bitcoin in their storage. the possibility of that happening is to me very slim because bitcoin will continue to circulate every day and the more people are ready to buy bitcoin, the more others will be willing to sell their bitcoin also which will keep the flow of bitcoin in circulation never ending.
before the 21 million bitcoins will be exhausted, some of the current holders will want to sell a portion to others who might be making use of it for daily trading purposes. bitcoin traders are always in the market and will continue to be ready for trading of bitcoin any day and anytime regardless of how scarce bitcoin becomes in the future due to basically an increase in it price. bitcoin is still in use as a payment option in different platforms and that in itself is going to keep bitcoin flowing across old and new users of bitcoin till the future that is coming.