But this is not sustainable. Imagine bitcoin prices continue to increase and eventually all the coins are mined. There are not many transfers because people are mainly hoarding. Thus not much transaction fees, which is the only source of income for miners. Hash power reduces dramatically, and so network security deteriorates.
Why not? People keep their wealth in a store of value (e.g., fiat currency savings, bonds, stocks, mutual funds, real estate, gold) and then sell those items as they need that wealth for spending.
If you have no limit of coins available the supply outweighs demand and the value goes down....and as the value goes down people realize that bitcoin is no longer a good option to store their wealth, and the whole system crashes. This is what fiat currency systems are driven by, that's why the spending power of your fiat money erodes over time, that's why regular commodities (things that have no increased tangible value, i.e., milk, eggs, gasoline) increases in cost over time.