At the long run, cryptocurrency will become more widely accepted, and it may even be used to buy groceries in supermarkets.
A long way to go from here. But we are hopeful of that in the future. Bitcoin currently has made a mark being a good investment, so I am not sure if that motion is going to get overruled back to its original use of currency.
The number of cryptocurrencies will decrease as more people utilize cryptocurrency.
If I understand correctly, you are mistaken. The total number of coins in circulation is fixed once mining of that coin ends minting all the coins. But it never reduces.
Scarcity gets more expensive as the quantity diminishes, and the price will automatically rise.
Scarcity occurs due to demand/supply gap. If people buy and dont sell, then it becomes scarce. But there is no method to remove coins from circulation unless people forget their private keys.
Again, quantity does not diminish with time. It remains fixed after having minted all coins. However the population being covered by that same number of fixed coins keeps on increasing.