once the last Bitcoin has been mined, who is going to provide the computing power to validate ownership of Bitcoins? The financial incentive to provide computing power will be gone. Who is going to prop up the "currency"?
they will still collect transaction fees
an equilibrium will establish.
because, if everyone stops mining, then CPU mining will once again be possible (because difficulty would lower) which means that mining will be very cheap and easy, which would lead to more people doing it.
eventually, the amount of miners will increase, causing the mining difficulty to rise again. as the mining difficulty rises, it gets more expensive, so some people will stop mining.
then mining difficulty will lower, meaning some people will start mining again....
and eventually, a nice happy medium will develop.
so, to put it simply, this is a non-issue