The total amount of coins ever is going to be 84 million.
It might go to 1000000$ per coin!
84 million is litecoin, for bitcoin it's 21 milloin.
Soon, when Bitcoin will pass $1000 people will use mBTC as it will be easier to talk about something near 1USD in value. It is not a mathematical problem or a technical problem, just our wetware limitations.
Don't forget that each bitcoin is not divided into 100 [per]cents like regular FIAT currrency, but into 100,000,000 sub units, so 1 millionth of a bitcoin still has 100 [per]cents and if and when 1BTC will reach the neighborhood of 1 Million USD people will switch from talking about mBTC to uBTC or the more accurate spelling: µBTC.
Inflation in USD makes cents not so useful and this leaves room for 0.01µBTC to be equal 1 USD.
If it gets so expensive so fast it will burst. Yes it might increase in value, but it shall happen within reasonable timeframe of years
What is your definition of "so fast"? It will take several years and even if it takes a decade it is still a good investment.