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 
You don't get it. There will always be someone doing the job, because if few people are doing it, the ones doing it will get a lot of reward. It gets itself balanced. Satoshi had it all planned.
I personally think that TX volume (and TX fees) will increase substantially over the years making the block subsidy less important