To some extent it's fallen off because we've reached (somewhat) of a plateau. Originally all mining was done using CPU resources, but the first big jump up in hashrate came as folks started to figure out GPU mining. As GPU mining grew more popular, the hard core geeks started looking at FPGAs and eventually we made it to full blown ASIC miners. The last big explosion of hashrate was driven by the bulk of miners adopting ASIC hardware of one flavor or another. Now that the majority of miners are on SHA256 ASICs the only gains to be made are by shrinking the die and increasing the transistor count but even those hard earned gains are going to be quickly diminished as ASIC hardware "catches up" to Moore's law.
As for hash power leaving the network, if you are mining as a business then you are 100% at the mercy of 1) how much you pay for electricity and 2) market price for coins. Given how many schemes exist for stealing either computing power or electricity I imagine the margins will converge on a negative number at a large enough scale.
As for hash power leaving the network, if you are mining as a business then you are 100% at the mercy of 1) how much you pay for electricity and 2) market price for coins. Given how many schemes exist for stealing either computing power or electricity I imagine the margins will converge on a negative number at a large enough scale.