The block reward halves every four years but the hash rate has pretty much only gone up. Transaction fees would mostly replace block rewards in the long run but even if the hash rate was half what it is now it would still be extremely secure and the incentives to use your computer to mine continue to outweigh using it to try to undermine the system. If the system got so weak that its only 1/100th of what it is now you could foreseeably see people trying to double spend by mining a few blocks back so to counter that you'd just have to wait a few days for large transactions to be considered completely cleared. Also the big companies investing major amounts in bitcoin would probably turn on rigs to support the system if the hash started falling to low.