When we reach the max supply. Miners will not get new Bitcoin, but they will get commissions that people will pay for including their transaction in blocks. Commissions must be quite high so that this work will still be interesting for miners, on the other hand, people will not use Bitcoin if it costs too much to transfer it. Maybe they will hold, but if nobody buys and sells, there is no movement in price, so it is an open question for me as well.