After all bitcoin is mined
Miner only get bitcoin only from tx fee
But, if bitcoin price never raise up
Only few people will mine it
Maybe bitcoin will add Aux-PoW / PoS / something else
Or hope someone want mine without get profit
It will be difficult for anyone to mine bitcoin
If bitcoin price is low
Miner only get bitcoin only from tx fee
But, if bitcoin price never raise up
Only few people will mine it
Maybe bitcoin will add Aux-PoW / PoS / something else
Or hope someone want mine without get profit
No one will be mining in 70 years unless Bitcoin is worth a ton of money. So it is do or die for the Bitcoin exchange rate. Otherwise we will never mine all of them.... unless we will have a difficulty decrease 
It will be difficult for anyone to mine bitcoin
If bitcoin price is low